aarch64: Use an aarch64-specific structure for vector costing

aarch64: Use an aarch64-specific structure for vector costing

This patch makes the AArch64 vector code use its own vector
costs structure, rather than just using the default unsigned[3].

Unfortunately, it's not easy to make this change specific to
use_new_vector_costs, so this part is one that affects all CPUs.
The change is relatively mechanical though.

gcc/
	* config/aarch64/aarch64.c (aarch64_vector_costs): New structure.
	(aarch64_init_cost): New function.
	(aarch64_add_stmt_cost): Use aarch64_vector_costs instead of
	the default unsigned[3].
	(aarch64_finish_cost, aarch64_destroy_cost_data): New functions.
	(TARGET_VECTORIZE_INIT_COST): Override.
	(TARGET_VECTORIZE_FINISH_COST): Likewise.
	(TARGET_VECTORIZE_DESTROY_COST_DATA): Likewise.