Build crashes on ios_armv7 with bazel

Hi, thank you for your great project.

I'd tried to build XNNPACK e2ebench with ios_armv7 config with commands below:
```shell
$ bazel build -c opt --config ios_armv7 :end2end_bench
```
and bazel build log saids:

console outputs

```shell
$ bazel build -c opt --config ios_armv7 :end2end_bench
INFO: Analyzed target //:end2end_bench (24 packages loaded, 1387 targets configured).
INFO: Found 1 target...
ERROR: /Users/jhyoo/workspace/src/XNNPACK/BUILD.bazel:3122:19: C++ compilation of rule '//:tables' failed (Exit 1): wrapped_clang failed: error executing command external/local_config_cc/wrapped_clang '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -g0 -O2 '-D_FORTIFY_SOURCE=1' -DNDEBUG ... (remaining 37 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox wrapped_clang failed: error executing command external/local_config_cc/wrapped_clang '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -g0 -O2 '-D_FORTIFY_SOURCE=1' -DNDEBUG ... (remaining 37 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox
clang: error: invalid iOS deployment version '--target=armv7-apple-ios', iOS 10 is the maximum deployment target for 32-bit targets [-Winvalid-ios-deployment-target]
Target //:end2end_bench failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 12.155s, Critical Path: 0.37s
INFO: 133 processes: 133 internal.
FAILED: Build did NOT complete successfully
```

The main reason seems ```clang: error: invalid iOS deployment version '--target=armv7-apple-ios', iOS 10 is the maximum deployment target for 32-bit ```. My ios sdk version is 13.7 so I guess local_config_cc put ```-miphoneos-version-min=13.7``` on ios_armv7 build. To avoid this I'd tried:

```
$ bazel build --ios_minimum_os='10.0' -c opt --config ios_armv7 :end2end_bench
```

And then I faced compile error because armv7 doesn't support dot product simd command:

console outputs

```shell
$ bazel build --ios_minimum_os='10.0' -c opt --config ios_armv7 :end2end_bench
INFO: Build option --ios_minimum_os has changed, discarding analysis cache.
INFO: Analyzed target //:end2end_bench (0 packages loaded, 1387 targets configured).
INFO: Found 1 target...
ERROR: /Users/jhyoo/workspace/src/XNNPACK/BUILD.bazel:3470:19: C++ compilation of rule '//:neondot_ukernels' failed (Exit 1): wrapped_clang failed: error executing command external/local_config_cc/wrapped_clang '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -g0 -O2 '-D_FORTIFY_SOURCE=1' -DNDEBUG ... (remaining 65 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox wrapped_clang failed: error executing command external/local_config_cc/wrapped_clang '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -g0 -O2 '-D_FORTIFY_SOURCE=1' -DNDEBUG ... (remaining 65 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:62:20: warning: implicit declaration of function 'vdotq_lane_s32' is invalid in C99 [-Wimplicit-function-declaration]
vacc0x0123 = vdotq_lane_s32(vacc0x0123, vb0123x0123, va0x01234567, 0);
^
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:62:18: error: assigning to 'int32x4_t' (vector of 4 'int32_t' values) from incompatible type 'int'
vacc0x0123 = vdotq_lane_s32(vacc0x0123, vb0123x0123, va0x01234567, 0);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:63:18: error: assigning to 'int32x4_t' (vector of 4 'int32_t' values) from incompatible type 'int'
vacc0x4567 = vdotq_lane_s32(vacc0x4567, vb0123x4567, va0x01234567, 0);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:64:18: error: assigning to 'int32x4_t' (vector of 4 'int32_t' values) from incompatible type 'int'
vacc0x0123 = vdotq_lane_s32(vacc0x0123, vb4567x0123, va0x01234567, 1);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:65:18: error: assigning to 'int32x4_t' (vector of 4 'int32_t' values) from incompatible type 'int'
vacc0x4567 = vdotq_lane_s32(vacc0x4567, vb4567x4567, va0x01234567, 1);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:79:20: warning: implicit declaration of function 'vdotq_lane_s32' is invalid in C99 [-Wimplicit-function-declaration]
vacc0x0123 = vdotq_lane_s32(vacc0x0123, vb0123x0123, va0x01234567, 0);
^
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:79:18: error: assigning to 'int32x4_t' (vector of 4 'int32_t' values) from incompatible type 'int'
vacc0x0123 = vdotq_lane_s32(vacc0x0123, vb0123x0123, va0x01234567, 0);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:80:18: error: assigning to 'int32x4_t' (vector of 4 'int32_t' values) from incompatible type 'int'
vacc0x4567 = vdotq_lane_s32(vacc0x4567, vb0123x4567, va0x01234567, 0);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:88:20: error: assigning to 'int32x4_t' (vector of 4 'int32_t' values) from incompatible type 'int'
vacc0x0123 = vdotq_lane_s32(vacc0x0123, vb4567x0123, va0x01234567, 1);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/qs8-gemm/gen/1x8c4-minmax-neondot.c:89:20: error: assigning to 'int32x4_t' (vector of 4 'int32_t' values) from incompatible type 'int'
vacc0x4567 = vdotq_lane_s32(vacc0x4567, vb4567x4567, va0x01234567, 1);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 warnings and 8 errors generated.
Target //:end2end_bench failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 7.145s, Critical Path: 6.64s
INFO: 215 processes: 130 internal, 85 darwin-sandbox.
FAILED: Build did NOT complete successfully
```

I'm not sure whether ios armv7 is still used popular or not but it was necessary for my work, so I fixed this by adding 'apple_aarch32_copt' on neon dot product targets in BUILD.bazel and add an ios option on ios_armv7 config.

If you're ok, may I make a PR for this?

Thank you. :)