arm-apple-none-macho uses DarwinTargetInfo which provides several Apple
specific macros. arm64-apple-none-macho however just uses the generic
AArch64leTargetInfo and doesn't get any of those macros. It's not clear
if everything from DarwinTargetInfo is desirable for
arm64-apple-none-macho, so make an AppleMachOTargetInfo to hold the
generic Apple macros and a few other basic things.
