[OpenMP][Tools] Fix Archer for MACOS

Archer uses weak symbol overloads of TSan functions to enable loading the tool
even if the application is not built with TSan. For MACOS the tool collects
the function pointer at runtime.
When adding the function entry/exit markers, we missed to add the functions
in the MACOS codepath.
This patch also replaces the repeated function lookup by a single initial
function lookup and fixes the disabling logic in RunningOnValgrind.

Differential Revision: https://reviews.llvm.org/D103607