AArch64: Add Neoverse N3 and Cortex-A725 core definition and cost model

AArch64: Add Neoverse N3 and Cortex-A725 core definition and cost model

This adds a cost model and core definition for Neoverse N3 and Cortex-A725.

It also makes Cortex-A725 use the Neoverse N3 cost model.

gcc/ChangeLog:

	* config/aarch64/aarch64-cores.def (neoverse-n3, cortex-a725): New.
	* config/aarch64/aarch64-tune.md: Regenerate.
	* config/aarch64/tuning_models/neoversen3.h: New file.
	* config/aarch64/aarch64.cc: Use it.
	* doc/invoke.texi: Document it.