R600: Use the same compute kernel calling convention for all GPUs

R600: Use the same compute kernel calling convention for all GPUs

A side-effect of this is that now the compiler expects kernel arguments
to be 4-byte aligned.

Reviewed-by: Vincent Lejeune <vljn at ovi.com>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186916 91177308-0d34-0410-b5e6-96231b3b80d8