Commit e1ce752a1d, "Cygwin: remove miscellaneous 32-bit code", removed
most occurrences of '#ifdef __x86_64__'. Restore those occurrences
that guarded code specific to the AMD64 processor, and #error out if
the processor is different. This will make it easier to find
AMD64-specific code if we ever want to add support for a different
64-bit processor (e.g., ARM64).