[AArch64] Fix a comment to match the code. NFC.

For the ELF case, the default/preferred form is the generic one, not
the short one as used for Apple - fix the comment to say so. Currently
it is a copy-paste typo.

Make the comments on the darwin default a bit more verbose.

Use enum names instead of literal 0/1 to further increase readability
and reduce fragility.

Differential Revision: https://reviews.llvm.org/D32963

llvm-svn: 302634