This is the first step towards a move of all builtin function
definitions living in TableGen. The intent is that once that is
accomplished, the storage cost and runtime cost (due to relocations) of
builtin function info structures can be significantly reduced by using
integer offsets into TableGen'd string tables rather than pointers.
The TableGen definitions live in a separate file to make future
conversions of the architecture-specific builtins more straightforward.
No functional change intended.