[darwin][driver] Pass through -global-isel LLVM flags to ld.

GlobalISel is currently not enabled when using -flto since the front-end
-mvllm flags don't get passed through. This change fixes this for Darwin
platforms. We have to do this in the driver because the code generator choice
isn't embedded into the bitcode file.

Differential Revision: https://reviews.llvm.org/D99126