Improvements to process affinity mask setting

A couple improvements:
1) Add ability to limit fullMask size when KMP_HW_SUBSET limits resources.
2) Make KMP_HW_SUBSET work for affinity_none, and only limit fullMask in this case.

Patch by Andrey Churbanov.

Differential Revision: http://reviews.llvm.org/D21528


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@273278 91177308-0d34-0410-b5e6-96231b3b80d8