[OMPT] Add missing ompt_get_num_procs function

This function is defined in OpenMP-TR6 section 4.1.5.1.6
The functions was not implemented yet.

Since ompt-functions can only be called after the runtime was initialized and
has loaded a tool, it can assume the runtime to be initialized. In contrast
to omp_get_num_procs which needs to check whether the runtime is initialized.

Differential Revision: https://reviews.llvm.org/D40949

llvm-svn: 321269