clang: Add __builtin_elementwise_rint and nearbyint

clang: Add __builtin_elementwise_rint and nearbyint

These are basically the same thing and only differ for strictfp,
so add both for future proofing. Note all the elementwise functions are
currently broken for strictfp, and use non-constrained ops. Add a test
that demonstrates this, but doesn't attempt to fix it.