[lld-macho] Set the SG_READ_ONLY flag on __DATA_CONST

This flag instructs dyld to make the segment read-only after fixups have
been performed.

I'm not sure why this flag is needed, as on macOS 13 beta at least,
__DATA_CONST is read-only even without this flag; but ld64 sets it as
well.

Differential Revision: https://reviews.llvm.org/D133010