[OpenMP] Handle function calls from `libomp` to `libomptarget` correctly

D132005 introduced function calls from `libomp` to `libomptarget` if offloading
is enabled. However, the external function declaration may not always work. For
example, it causes a link error on macOS. Currently it is guarded properly by
a macro, but in order to get OpenMP target offloading working on macOS, it has
to be handled correctly. This patch applies the same idea of how we support
target memory extension by using function pointer indirect call for that function.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D149557