KMP_HW_SUBSET vs KMP_PLACE_THREADS rival envirables fix

If both KMP_HW_SUBSET and KMP_PLACE_THREADS are set and KMP_PLACE_THREADS gets
parsed first, then the current environment variable parser rejects both and
neither get used. This patch uses the rivals mechanism that is used for other
environment variable groups (e.g., KMP_STACKSIZE, GOMP_STACKSIZE, OMP_STACKSIZE).
If both are set, then it tells the user that it is ignoring KMP_PLACE_THREADS in
favor of KMP_HW_SUBSET. The message about deprecating KMP_PLACE_THREADS when it
is set is still printed regardless.

Differential Revision: https://reviews.llvm.org/D38292

llvm-svn: 315091