Use Clang + aarch64 configuration to test library in aarch64 mode

I don't currently have a setup to test aarch64 with GNU, but I should be able to with clang.

We should test aarch64, aarch64_32, and aarch64_be to make sure all the proper configurations work right and are warning free.

I also see arm64/arm64_32/arm_b - are these just aliases?

$ llc --version
LLVM (http://llvm.org/):
LLVM version 10.0.0
Optimized build.
Default target: x86_64-apple-darwin18.7.0
Host CPU: haswell

Registered Targets:
aarch64 - AArch64 (little endian)
aarch64_32 - AArch64 (little endian ILP32)
aarch64_be - AArch64 (big endian)
amdgcn - AMD GCN GPUs
arm - ARM
arm64 - ARM64 (little endian)
arm64_32 - ARM64 (little endian ILP32)
armeb - ARM (big endian)