i386: Mark XMM4-XMM6 as clobbered by encodekey128/encodekey256
encodekey128 and encodekey256 operations clear XMM4-XMM6. But it is
documented that XMM4-XMM6 are reserved for future usages and software
should not rely upon them being zeroed. Change encodekey128 and
encodekey256 to clobber XMM4-XMM6.