[OpenMP][AMDGPU] Add 'uniform-work-group' attribute to OpenMP kernels

The `cl-uniform-work-group` attribute asserts that the global work-size
be a multiple of the work-group specified work group size. This should
allow optimizations. It is already present by default in the AMD
compiler and for HIP kernels so it should be safe to allow this for
OpenMP kernels by default.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D135374