Add AArch64 (64-bit ARM) target support

* include/private/gcconfig.h (AARCH64): New macro (defined only if
__aarch64__).
* include/private/gcconfig.h (mach_type_known): Update comment adding
ARM AArch64 target.
* include/private/gcconfig.h (NOSYS, mach_type_known, CPP_WORDSZ,
MACH_TYPE, ALIGNMENT, HBLKSIZE, OS_TYPE, LINUX_STACKBOTTOM,
DYNAMIC_LOADING, DATASTART, DATAEND, STACKBOTTOM): Define for AArch64.
Update AUTHORS file