[OpenMP][libomptarget] Add runtime function for pushing coalesced global records

Summary: In the case of coalesced global records, we need to push the exact data size passed in. This patch fixes this by outlining the common functionality of the previous push function and by adding a separate entry point for coalesced pushes. The pop function remains unchanged.

Reviewers: ABataev, grokos, caomhin

Reviewed By: ABataev, grokos

Subscribers: jholewinski, cfe-commits, Hahnfeld, guansong, jfb, openmp-commits

Differential Revision: https://reviews.llvm.org/D53141

llvm-svn: 345867