[OpenMP] Only enable version script if supported

The linker flag `--version-script` may not be supported by all linkers, such as
macOS's linker. `libomp` is already capable of detecting whether the linker supports
it and append the linker flag accordingly. Since currently we assume `libomptarget`
only works on Linux, we don't do the check accordingly. This patch simply adds
the check before adding it to linker flag. This will be the first patch to make
OpenMP target offloading work on macOS. Note that CMake files in `plugins` are
not touched before they are going to be removed pretty soon anyway.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D149555