R600: Update GPU variants in -mcpu option

We've added the RS880 variant in the LLVM backend to represent an R600
GPU with no vertex cache, so we need to update the GPU mappings for
-mcpu.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181202 91177308-0d34-0410-b5e6-96231b3b80d8