[lld/mac] Make framework symlinks in tests more realistic

In a framework Foo.framework, Foo.framework/Foo is usually a relative
symbolic link to Foo.framework/Versions/Current/Foo,
and Foo.framework/Versions/Current is usually a relative symbolic
link to A.

Our tests used absolute symbolic links. Now they use relative symbolic links.

No behavior change, just makes the tests more representative of the real world.

(implicit-dylib.s omits the "Current" folder too, but I'm not changing that
here.)

Differential Revision: https://reviews.llvm.org/D103998