AMDGPU: Start defining a calling convention

AMDGPU: Start defining a calling convention

Partially implement callee-side for arguments and return values.
byval doesn't work properly, and most likely sret or other on-stack
return values most as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303308 91177308-0d34-0410-b5e6-96231b3b80d8