R600/SI: Start implementing an assembler

This was done using the Sparc and PowerPC AsmParsers as guides.  So far it
is very simple and only supports sopp instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221994 91177308-0d34-0410-b5e6-96231b3b80d8