[libunwind] Add UNW_AARCH64_* beside UNW_ARM64_*

The original libunwind project defines UNW_AARCH64_* instead of UNW_ARM64_*.
Rename the enum members to match. This allows some applications with simple
`unw_init_local` usage to migrate to llvm-project libunwind.

Note: the canonical names of `UNW_ARM_D{0..31}` are now `UNW_AARCH64_V{0..31}`,
to match the original libunwind.

UNW_ARM64_* are kept for now for compatibility. Some may be unneeded and can be
cleaned up in the future.

Reviewed By: #libunwind, compnerd

Differential Revision: https://reviews.llvm.org/D107996