[AMDGPU] Add missing `__builtin_amdgcn_wavefrontsize` builtin (#80741)

Summary:
The backend supports the wavefrontsize intrinsic, and suggests that it
is tied to a corresponding clang builtin, but it is not actually
present. This simply adds it in so it can be used from clang. This
attribute likely isn't the best to rely on, but for the `libc` use-case
we will need to detect a struct's differing size in a way that will
depend on the wavefront size.