Support external toolchains

Currently, gcc-cross-${TARGET_ARCH} is hard-coded, resulting in failure if something like external-arm-toolchain is used instead:

ERROR: Nothing PROVIDES 'gcc-cross-arm' (but /home/jon/src/jmagnuson/yocto/rpi0/layers/meta-rust-bin/recipes-devtools/rust/rust-bin-cross_1.44.0.bb DEPENDS on or otherwise requires it)
gcc-cross-arm was skipped: PREFERRED_PROVIDER_virtual/arm-none-linux-gnueabihf-gcc set to external-arm-toolchain, not gcc-cross-arm
gcc-cross-arm was skipped: PREFERRED_PROVIDER_virtual/arm-none-linux-gnueabihf-gcc set to external-arm-toolchain, not gcc-cross-arm
gcc-cross-arm was skipped: PREFERRED_PROVIDER_virtual/arm-none-linux-gnueabihf-gcc set to external-arm-toolchain, not gcc-cross-arm
gcc-cross-arm was skipped: PREFERRED_PROVIDER_virtual/arm-none-linux-gnueabihf-gcc set to external-arm-toolchain, not gcc-cross-arm
NOTE: Runtime target 'hello-rust' is unbuildable, removing...
Missing or unbuildable dependency chain was: ['hello-rust', 'cargo-bin-cross-arm', 'rust-bin-cross-arm', 'gcc-cross-arm']
ERROR: Required build target 'packagegroup-rpi-test' has no buildable providers.
Missing or unbuildable dependency chain was: ['packagegroup-rpi-test', 'hello-rust', 'cargo-bin-cross-arm', 'rust-bin-cross-arm', 'gcc-cross-arm']

Patching directly works:

diff --git a/recipes-devtools/rust/rust-bin-cross.inc b/recipes-devtools/rust/rust-bin-cross.inc
index 25ca590..0145503 100644
--- a/recipes-devtools/rust/rust-bin-cross.inc
+++ b/recipes-devtools/rust/rust-bin-cross.inc
@@ -7,7 +7,7 @@ inherit cross
 inherit rust-common

 # Required to link binaries
-DEPENDS += "gcc-cross-${TARGET_ARCH}"
+DEPENDS += "external-${TARGET_ARCH}-toolchain"

 PN = "rust-bin-cross-${TARGET_ARCH}"

but the long-term solution would be to determine which to pull in at build time. Checking the environment for TCMODE or EXTERNAL_TOOLCHAIN could work, or possibly just using virtual/${TARGET_PREFIX}gcc itself.

Start in recipes-devtools/rust/rust-bin-cross.inc, where the cross-toolchain dependency is hard-coded to gcc-cross-${TARGET_ARCH}. Review the issue's alternatives involving TCMODE, EXTERNAL_TOOLCHAIN, or virtual/${TARGET_PREFIX}gcc, then validate the result by building with external-arm-toolchain without the gcc-cross-arm provider error.