[OpenMP] Introduce '-f[no-]openmp-target-jit' flag to control JIT for offloading

JIT support for OpenMP offloading was introduced in D139287. This patch
adds a simple flag that enables this mode. It simply requires enabling
`-foffload-lto` mode and `--embed-bitcode` in the linker wrapper. This
option implies LTO if it is not enabled.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D141158