Unwind: tweak register handling for AArch64

AArch64 uses // as the comment character (although, Darwin uses ;).  However,
since we are using the C preprocessor on these files, // can be used as the
comment character across the board.

Tweak the platform guard to recognise __aarch64__ as well as __arm64__ for the
platform identifier.

llvm-svn: 231578