[Libomptarget] Only export needed definitions in the BC library

This patch adds the use of the `-internalize-public-api-file` option in
the internalization pass to internalize any definition that isn't
explicitly needed for the interface. This will allow us to perform more
optimizations on the file that normally would not have been possible
with functions internal to the library not being internal.

Depends on D130293

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D130298