[AMDGPU] Rename predefined macro __AMDGCN_WAVEFRONT_SIZE

rename it to __AMDGCN_WAVEFRONT_SIZE__ for consistency.

__AMDGCN_WAVEFRONT_SIZE will be deprecated in the future.

Reviewed by: Matt Arsenault, Johannes Doerfert

Differential Revision: https://reviews.llvm.org/D154207