[OpenMP][libomp] Parameterize affinity functions

This patch parameterizes the affinity initialization code to allow multiple
affinity settings. Almost all global affinity settings are consolidated
and put into a structure kmp_affinity_t. This is in anticipation of the
addition of hidden helper affinity which will have the same syntax and
semantics as KMP_AFFINITY only for the hidden helper team.

Differential Revision: https://reviews.llvm.org/D135109