[Libomptarget] Embed bitcode library in static library instead.

This patch changes the CMake to instead embed the already generated
LLVM-IR bitcode library into an object file to create the static
library. This is different from the previous method which generated them
separately. This will make the build faster and allow us to perform the
same internalization into a single library we do with the bitcode
library.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D133952