[pstl] Rename PARALLELSTL_BACKEND to PSTL_PARALLEL_BACKEND

It makes more sense to name configuration options as PSTL_XXX.
Also, I'm naming it PSTL_PARALLEL_BACKEND because we might introduce
the ability to customize the vectorization backend, in which case
PSTL_BACKEND would become ambiguous.

llvm-svn: 368672