[Driver][test] Refactor LLVMgold tests

LLVMgold.so tests are duplicated in several places. Deduplicate them.
Move the tests to lto.c and lto.cu
Specify -fuse-ld=bfd or -fuse-ld=gold.

In a future change, if -fuse-ld=lld or CLANG_DEFAULT_LINKER=lld without -fuse-ld=, we will remove -plugin /path/to/LLVMgold.so