[OpenMP][AMDGCN] Initial math headers support

With this patch, OpenMP on AMDGCN will use the math functions
provided by ROCm ocml library. Linking device code to the ocml will be
done in the next patch.

Reviewed By: JonChesterfield, jdoerfert, scchan

Differential Revision: https://reviews.llvm.org/D104904