[OpenMP] Emit calls to int64_t functions for amdgcn

[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