[AMDGPU] calling conventions for AMDPAL OS type

Summary:
This commit adds comments on how the AMDPAL OS type overloads the
existing AMDGPU_ calling conventions used by Mesa, and adds a couple of
new ones.

Reviewers: arsenm, nhaehnle, dstuttard

Subscribers: mehdi_amini, kzhuravl, wdng, yaxunl, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D37752

llvm-svn: 314502