Build issues on an AArch32 userspace when running an AArch64 kernel

Trying to build on a system running Linux 3.18.0 (AArch64) in an AArch32 chroot (Arch Linux ARM, gcc 6.3.1, cmake 3.7.2). The first error I get is:

```
[ 11%] Building ASM object dynamorio/core/CMakeFiles/drhelper.dir/arch/asm_shared.asm.o
In file included from /home/cosmin/drmemory/dynamorio/core/arch/asm_shared.asm:37:0:
/home/cosmin/drmemory/dynamorio/core/arch/asm_defines.asm:65:3: error: #error ARM is only 32-bit; AARCH64 is 64-bit
# error ARM is only 32-bit; AARCH64 is 64-bit
^~~~~
```

I guessed that the architecture reported by the kernel is used at some point in the build process, so I've then enabled the PER_LINUX32 personality by prepending `linux32` to the `cmake` and `make` commands (on a clean build). This changes the output of `uname`, `/proc/cpuinfo`, etc to be compatible with that on AArch32 kernels. The build then completed.

DynamoRIO `266b8a3c8b` on its own also fails to build without PER_LINUX32, but with a different error, so I think both the DR and Dr. Memory build systems misbehave.

```
[ 8%] Building CXX object clients/drcachesim/CMakeFiles/drcachesim_ops.dir/common/options.cpp.o
c++: error: unrecognized command line option '-mpreferred-stack-boundary=2'; did you mean '-mstructure-size-boundary='?
```

I've generally found the output of `$(CC) -dumpmachine` to be a more robust way of detecting the target architecture, but I'm not sure how well that would work with your build system.