[lld/mac] Use libSystem.dylib instead of libSystem.B.dylib in tests

For -flat_namespace, lld needs to load dylibs in LC_LOAD_DYLIB.
The current setup meant that libSystem.dylib would cause a LC_LOAD_DYLIB
with libSystem.B.dylib, but that didn't exist in our libsysroot for
tests. So just drop the .B.

See https://reviews.llvm.org/D97641#2595237 and
https://reviews.llvm.org/D97641#2595270