[OpenMP][libomptarget] Enable multiple frames per global memory slot

Summary: To save on calls to malloc, this patch enables the re-use of pre-allocated global memory slots.

Reviewers: ABataev, grokos, carlo.bertolli, caomhin

Reviewed By: grokos

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D44470

llvm-svn: 327637