[OpenMP][JIT] Fixed a couple of issues in the initial implementation of JIT

This patch fixes a couple of issues:
1. Instead of using `llvm_unreachable` for those base virtual functions, unknown
   value will be returned. The previous method could cause runtime error for those
   targets where the image is not compatible but JIT is not implemented.
2. Fixed the type in CMake that causes the `Target` CMake variable is undefined.

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D140732