[CMake] Let llvm-shlib work on Linux with --whole-archive.

FIXME: It should work on not only Linux but elf-targeting gnu ld.

For example if LLVM_DYLIB_COMPONENTS is "BitWriter Support", CMake emits the command line like;

  -Wl,--whole-archive
    lib/libLLVMBitWriter.a
    lib/libLLVMSupport.a *1
  -Wl,--no-whole-archive
  lib/libLLVMCore.a
  lib/libLLVMSupport.a   *2
  -lrt -ldl -ltinfo -lpthread -lm

It works since symbols in LLVMCore is resolved with not *2 but *1.

Unfortunately, --gc-sections is not powerful in this case to prune unused "visibility(default)" entries.

I am still experimenting other way not to rely on --whole-archive.
[CMake] llvm-c-test: Use libLLVM.so if it is available.