Deprecate KMP_PLACE_THREADS and rename as KMP_HW_SUBSET

Deprecate KMP_PLACE_THREADS and rename it to KMP_HW_SUBSET due to confusion
about its purpose and function among users.  KMP_HW_SUBSET is an environment
variable which allows users to easily pick a subset of the hardware topology to
use.  e.g., KMP_HW_SUBSET=30c,2t means use 30 cores, 2 threads per core.

Patch by Andrey Churbanov

Differential Revision: http://reviews.llvm.org/D21340

llvm-svn: 272937