[OpenMP] Emit calls to int64_t functions for amdgcn
Two functions, syncwarp and active_thread_mask, return lanemask_t. Currently
this is assumed to be int32, which is true for nvptx. Patch makes the type
target architecture dependent.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D89746
Jon Chesterfield