[Patch AArch64 Obvious] Unify branch costs to generic_branch_cost
All the cores in AArch64 use the pair {1, 3} for their branch costs. As
that is covered by generic_branch_cost, we can just use that directly and
save the tiny amount of redundant code. If in future any core wants to
modify this, they can always add a special-case branch-cost back.