rs6000: Remove builtin mask check from builtin_decl [PR102347]
As the discussion in PR102347, currently builtin_decl is invoked so
early, it's when making up the function_decl for builtin functions,
at that time the rs6000_builtin_mask could be wrong for those
builtins sitting in #pragma/attribute target functions, though it
will be updated properly later when LTO processes all nodes.
This patch is to align with the practice i386 port adopts, also
align with r10-7462 by relaxing builtin mask checking in some places.