AArch64: Add new generic-armv8-a CPU and make it the default.

AArch64: Add new generic-armv8-a CPU and make it the default.

This patch adds a new generic scheduling model "generic-armv8-a" and makes it
the default for all Armv8 architectures.

-mcpu=generic and -mtune=generic is kept around for those that really want the
previous cost model.

This shows on SPECCPU 2017 the following:

generic:  SPECINT 1.0% improvement in geomean, SPECFP -0.6%.  The SPECFP is due
          to fotonik3d_r where we vectorize an FP calculation that only ever
	  needs one lane of the result.  This I believe is a generic costing bug
	  but at the moment we can't change costs of FP and INT independently.
	  So will defer updating that cost to stage3 after Richard's other
	  costing updates land.

generic SVE: SPECINT 1.1% improvement in geomean, SPECFP 0.7% improvement.

gcc/ChangeLog:

	PR target/111370
	* config/aarch64/aarch64-arches.def (armv8-9, armv8-a, armv8.1-a,
	armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv8.7-a,
	armv8.8-a): Update to generic_armv8_a.
	* config/aarch64/aarch64-cores.def (generic-armv8-a): New.
	* config/aarch64/aarch64-tune.md: Regenerate.
	* config/aarch64/aarch64.cc: Include generic_armv8_a.h
	* config/aarch64/aarch64.h (TARGET_CPU_DEFAULT): Change to
	TARGET_CPU_generic_armv8_a.
	* config/aarch64/tuning_models/generic_armv8_a.h: New file.

gcc/testsuite/ChangeLog:

	PR target/111370
	* gcc.target/aarch64/sve/cond_asrd_1.c: Updated.
	* gcc.target/aarch64/sve/cond_cnot_4.c: Likewise.
	* gcc.target/aarch64/sve/cond_unary_5.c: Likewise.
	* gcc.target/aarch64/sve/cond_uxt_5.c: Likewise.
	* gcc.target/aarch64/target_attr_13.c: Likewise.
	* gcc.target/aarch64/target_attr_15.c: Likewise.