[AMDGPU] AMDGPUUsage define call convention ABI

Reviewers: scott.linder, arsenm, b-sumner

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74861