Add arc defaults for onto branch
This indicates to arc that main is the default branch onto which to
land changes. It includes both the older "arc.land.onto.default" and
newer "arc.land.onto" spellings of this option to accomodate users with
various versions of arc. See also https://reviews.llvm.org/D97215.
Reviewed By: mehdi_amini
Differential Revision: https://reviews.llvm.org/D97216