encodeUtf8_2: add fast paths for x86_64 and i386
encodeUtf8_2: add fast paths for x86_64 and i386

This helps performance a lot in most cases: up to 2x faster, in
fact. The exception seems to be Japanese, which is slowed down by
about 10%.

bos/text,hvr/text,spencerjanssen/text,beni55/text,spencerjanssen/text,23Skidoo/text,bgamari/text,text-utf8/text,bgamari/text,text-utf8/text,hvr/text,ekmett/text,bgamari/text,ekmett/text,bos/text,alexliew/text,hvr/text,beni55/text,aloiscochard/text,23Skidoo/text,bos/text,ekmett/text,beni55/text,spencerjanssen/text,ekmett/text,23Skidoo/text,23Skidoo/text,alexliew/text,aloiscochard/text,bos/text,aloiscochard/text,text-utf8/text,beni55/text,hvr/text,alexliew/text,aloiscochard/text,text-utf8/text,spencerjanssen/text,alexliew/text,bgamari/text