[lld/mac] When handling @loader_path, use realpath() of symlinks

This is important for Frameworks, which are usually symlinks.

ld64 gets this right for @rpath that's replaced with @loader_path, but not for
bare @loader_path -- ld64's code calls realpath() in that case too, but ignores
the result.

ld64 somehow manages to find libbar1.dylib in the test without the
explicit `-rpath` in Foo1. I don't understand why or how. But this
change is a step forward and fixes an immediate problem I'm having,
so let's start with this :)

Differential Revision: https://reviews.llvm.org/D103990