[Patch AArch64 Obvious] Unify branch costs to generic_branch_cost

[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.

gcc/

	* config/aarch64/aarch64.c (cortexa57_branch_cost): Remove.
	(thunderx2t99_branch_cost): Likewise.
	(cortexa35_tunings): Update to use generic_branch_cost.
	(cortexa53_tunings): Likewise.
	(cortexa57_tunings): Likewise.
	(cortexa72_tunings): Likewise.
	(cortexa73_tunings): Likewise.
	(thunderx2t99_tunings): Likewise.

From-SVN: r250582