[lld-macho][NFC] define more strings in section_names:: and segment_names::

As preparation for a subsequent diff that implements builtin section renaming, define more `constexpr` strings in namespaces `lld::macho::segment_names` and `lld::macho::section_names`, and use them to replace string literals.

Differential Revision: https://reviews.llvm.org/D101393