[lld-macho] Flip ZERO_AR_DATE default

Previously unless ZERO_AR_DATE was set to any value, ld64.lld would
embed non-hermetic timestamps into the final binary. This flips that
default to zero those values unless ZERO_AR_DATE is set explicitly to 0.
As far as I know there isn't a downside to this, except that it differs
from ld64.

Differential Revision: https://reviews.llvm.org/D135529