[libunwind] Clean up __arm64__

`__aarch64__` is defined for the target (since the beginning of arm64 support: clang 3.5).
`__arm64__` is only defined for the Darwin OS on AArch64.

`defined(__aarch64__) || defined(__arm64__)` can be simplied as `defined(__aarch64__)`

Darwin AArch64 uses %% as the assembly separator (see AArch64MCAsmInfo.cpp).
Make the intention explicit in src/assembly.h

With this change, the libunwind code base has no reference of `__arm64__`/`__arm64`.

Reviewed By: #libunwind, ldionne, mstorsjo

Differential Revision: https://reviews.llvm.org/D77829