Make affinity support conditional on KMP_AFFINITY_SUPPORTED

The feature was previously guarded with KMP_OS_LINUX || KMP_OS_WINDOWS but can
now be enabled/disabled independently to simplify porting.

Completes the work started in r202478.

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@202613 91177308-0d34-0410-b5e6-96231b3b80d8
Testing commit hook.


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@202867 91177308-0d34-0410-b5e6-96231b3b80d8
Revert commit (testing commit hook).


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@202868 91177308-0d34-0410-b5e6-96231b3b80d8