[LLVM][compiler-rt][AArch64] Refactor AArch64 CPU features

[boomanaiden154]

This patch refactors the AArch64 CPUFeatures enum into a separate include file that is identical between LLVM and compiler-rt. This, along with a test in compiler-rt to ensure that the two stay in sync.

[tmatheson-arm]

This looks like an improvement, but I'm still not sure why we couldn't just include the same .inc file from two different places.

[boomanaiden154]

This looks like an improvement, but I'm still not sure why we couldn't just include the same .inc file from two different places.

I don't believe the out-of-tree builds matter anymore with a refactoring of the runtimes build a while back (but I could be wrong). @MaskRay would probably have better context on any technical reasons why. The licensing constraints also might not want to be something we want to touch, although I would think copying and pasting selected .inc files would incur the same restraints as just copying it over at build time.

Anyways, I definitely think this is an improvement. Even if unifying the .inc files is actually viable, it's another trivial patch after this one (delete the test and one of the .inc files), made slightly easier by the refactoring in this patch.

[boomanaiden154]

I'll land this. I'll see if I have some time later to take a look at the CMake solution mentioned in https://discourse.llvm.org/t/code-sharing-between-compiler-rt-and-clang-llvm/79935/9?u=boomanaiden154-1 so that we can actually share the code rather than just maintaining identical copies.