The AArch64 backend sets BRANCH_COST to be the constant value 2 for all cpus,
meaning that the compiler thinks that branches cost the same across all cpus.

This patch reworks the handling of branch costs to allow per-cpu values to be
set. The actual value of the branch-costs is unchanged as the correct values for
will need to be decided for each core.

Tested aarch64-none-linux-gnu with gcc-check.

Ok for trunk?
Matthew

2015-05-21  Matthew Wahab  <matthew.wahab@arm.com>

	* 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.