[lld/macho] Fixes the -ObjC flag

When checking the segment name for Swift symbols, we should be checking that they start with `__swift` instead of checking for equality

Fixes the issue https://github.com/llvm/llvm-project/issues/55355

Reviewed By: #lld-macho, keith, thevinster

Differential Revision: https://reviews.llvm.org/D125250