[OpenMP][libomp] Add use-all syntax to KMP_HW_SUBSET

This patch allows the user to request all resources of a particular
layer (or core-attribute). The syntax of KMP_HW_SUBSET is modified
so the number of units requested is optional or can be replaced with an
'*' character.

e.g., KMP_HW_SUBSET=c:intel_atom@3 will use all the cores after offset 3
e.g., KMP_HW_SUBSET=*c:intel_core will use all the big cores
e.g., KMP_HW_SUBSET=*s,*c,1t will use all the sockets, all cores per
      each socket and 1 thread per core.

Differential Revision: https://reviews.llvm.org/D115826