AMDGPU/clang: Add builtins for llvm.amdgcn.ballot

Use explicit _w32/_w64 suffixes for the wave size to be consistent
with the existing other wave dependent intrinsics. Also start
diagnosing trying to use both wave32 and wave64.

I would have preferred to avoid the +wavefrontsize64 spam on targets
where that's the only option, but avoiding this seems to be more work
than I expected.