[Triple] Add AMDPAL operating system type

Summary:
This operating system type represents the AMDGPU PAL runtime, and will
be required by the AMDGPU backend in order to generate correct code for
this runtime.

Currently it generates the same code as not specifying an OS at all.
That will change in future commits.

Patch from Tim Corringham.

Subscribers: arsenm, nhaehnle

Differential Revision: https://reviews.llvm.org/D37380

llvm-svn: 314500