[AArch64] Add branch-cost to cpu tuning information.

[AArch64] Add branch-cost to cpu tuning information.

	* gcc/config/aarch64-protos.h (struct cpu_branch_cost): New.
        (tune_params): Add field branch_costs.
	(aarch64_branch_cost): Declare.
	* gcc/config/aarch64.c (generic_branch_cost): New.
	(generic_tunings): Set field cpu_branch_cost to generic_branch_cost.
	(cortexa53_tunings): Likewise.
	(cortexa57_tunings): Likewise.
	(thunderx_tunings): Likewise.
	(xgene1_tunings): Likewise.
	(aarch64_branch_cost): Define.
	* gcc/config/aarch64/aarch64.h (BRANCH_COST): Redefine.

From-SVN: r222805