[Libomptarget] Implement a unified kernel entry function

This patch implements a unified kernel entry function that will be
targeted from both teams and non-teams clauses. We introduce a new
interface and make the old functions call in using the new one. A
following patch will include the necessary changes to Clang to call
these new functions instead.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D128549