[OpenMP][Clang] Add call to global data sharing stack initialization on the workers side

Summary: The workers also need to initialize the global stack. The call to the initialization function needs to happen after the kernel_init() function is called by the master. This ensures that the per-team data structures of the runtime have been initialized.

Reviewers: ABataev, grokos, carlo.bertolli, caomhin

Reviewed By: ABataev

Subscribers: jholewinski, guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D44749

llvm-svn: 328219