[OpenMP] Introduce GOMP teams support in runtime

Implement GOMP_teams_reg() function which enables GOMP support of the
standalone teams construct. The GOMP_parallel* functions were modified
to call __kmp_fork_call() unconditionally so that the teams-specific
code could be reused within __kmp_fork_call() instead of reproduced
inside the GOMP_* functions.

Differential Revision: https://reviews.llvm.org/D87167