[lld-macho] Preserve alignment for non-deduplicated cstrings

Fixes PR50637.

Downstream bug: https://crbug.com/1218958

Currently, we split __cstring along symbol boundaries with .subsections_via_symbols
when not deduplicating, and along null bytes when deduplicating. This change splits
along null bytes unconditionally, and preserves original alignment in the non-
deduplicated case.

Removing subsections-section-relocs.s because with this change, __cstring
is never reordered based on the order file.

Differential Revision: https://reviews.llvm.org/D104919