AArch64: Add baseline tune

AArch64: Add baseline tune

Cleanup the extra tune defines by introducing AARCH64_EXTRA_TUNE_BASE as a
common base supported by all modern cores.  Initially set it to
AARCH64_EXTRA_TUNE_CHEAP_SHIFT_EXTEND.  No change in generated code.

gcc:
	* config/aarch64/aarch64-tuning-flags.def (AARCH64_EXTRA_TUNE_BASE): New define.
	* config/aarch64/tuning_models/ampere1b.h: Use AARCH64_EXTRA_TUNE_BASE.
	* config/aarch64/tuning_models/cortexx925.h: Likewise.
	* config/aarch64/tuning_models/fujitsu_monaka.h: Likewise.
	* config/aarch64/tuning_models/generic_armv8_a.h: Likewise.
	* config/aarch64/tuning_models/generic_armv9_a.h: Likewise.
	* config/aarch64/tuning_models/neoversen1.h: Likewise.
	* config/aarch64/tuning_models/neoversen2.h: Likewise.
	* config/aarch64/tuning_models/neoversen3.h: Likewise.
	* config/aarch64/tuning_models/neoversev1.h: Likewise.
	* config/aarch64/tuning_models/neoversev2.h: Likewise.
	* config/aarch64/tuning_models/neoversev3.h: Likewise.
	* config/aarch64/tuning_models/neoversev3ae.h: Likewise.