[StrTable] Add prefixes for x86 builtins.
This requires adding support to the general builtins emission for
producing prefixed builtin infos separately from un-prefixed which is
a bit crufty. But we don't currently have any good way of having a more
refined model than a single hard-coded prefix string per TableGen
emission. Something more powerful and/or elegant is possible, but this
is a fairly minimal first step that at least allows factoring out the
builtin prefix for something like X86.
