[lld-macho] Align all cstrings to 16 bytes when deduplicating

We previously did this only for x86_64, but it turns out that
arm64 needs this too -- see PR50791.

Ultimately this is a hack, and we should avoid over-aligning strings
that don't need it. I'm just having a hard time figuring out how ld64 is
determining the right alignment.

No new test for this since we were already testing this behavior for
x86_64, and extending it to arm64 seems too trivial.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D104835