Add --funified-lto command-line option, same as clang.

[JohanEngelen]

Resolves #5047

[JohanEngelen]

the test for now intentionally does not use -funified-lto, to see whether it is actually really needed when mixing full and thin lto object files. Apparently not needed on my macbook...

[kinke]

Yeah, that used to work just fine at least (I remember suggesting to use -flto=thin to create static libs which can then be used for both full and thin LTO). Plus lib{druntime,phobos}-lto are compiled with -flto=thin, and linked into the official builds using -flto=full for the compiler parts, so should be tested too.