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

llvm-svn: 273278