[OpenMP] Restore backwards compatibility for libomptarget

Summary:
The changes introduced in D87946 changed the API for libomptarget
functions. `__kmpc_push_target_tripcount` was a function in Clang 11.x
but was not given a backward-compatible interface. This change will
require people using Clang 13.x or 12.x to recompile their offloading
programs.

Reviewed By: jdoerfert cchen

Differential Revision: https://reviews.llvm.org/D98358

(cherry picked from commit 807466ef)