[libomptarget] Implement smid for amdgcn

Summary:
[libomptarget] Implement smid for amdgcn

Implementation is in a new file as it uses an intrinsic with
complicated encoding that warranted substantial comments.

Reviewers: jdoerfert, grokos, ABataev, ronlieb

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D72956