[OpenMP] Always pass the optimization level to the linker wrapper

The linker wrapper runs LTO internally, so it needs to know the
optimization level the user requested, if any. Previously this was only
done in `-foffload-lto` mode as we were assuming that this would enble
LTO. However, AMDGPU always performs LTO, and it's possible to run clang
on object files to link without passing this flag. So we should just
respect it always.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D158298