Triple: Add AMDHSA operating system type

This operating system type represents the AMD HSA runtime,
and will be required by the R600 backend in order to generate
correct code for this runtime.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223124 91177308-0d34-0410-b5e6-96231b3b80d8