x86-64: limit breakage from gcc movdir64b et al workaround

x86-64: limit breakage from gcc movdir64b et al workaround

This is only a partial fix for PR/gas 27419, in that it limits the bad
behavior of accepting mismatched operands to just x32 mode. The full fix
would be to revert commits 27f134698ac5 and b3a3496f83a1, and to address
the issue in gcc instead.