[llvm] r221591 - [CMake] Let llvm-shlib work on Linux with --whole-archive.

[Rafael Espíndola]

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

This is very likely just a bug on old versions of bfd ld. Version
2.23.1 and newer should work. See r220237.

Cheers,
Rafael