[OpenMP] Remove declaration and usage of __AMDGCN_WAVEFRONT_SIZE (#143761)

Removes usage of __AMDGCN_WAVEFRONT_SIZE as compile time constant.

---------

Co-authored-by: Shilei Tian <i@tianshilei.me>
GitOrigin-RevId: cf6ae065a042aae6324b28e99628c40bc53be0b7
1 file changed