[OpenMP] Make the exec_mode global have protected visibility

We use protected visibility for almost everything with offloading. This
is because it provides us with the ability to read things from the host
without the expectation that it will be preempted by a shared library
load, bugs related to this have happened when offloading to the host.
This patch just makes the `exec_mode` global generated for each plugin
have protected visibility.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D135285