[AArch64] [-mtune cleanup 2/5] Tune for Cortex-A53 by default.

[AArch64] [-mtune cleanup 2/5] Tune for Cortex-A53 by default.

gcc/
	* config/aarch64/aarch64-arches.def (armv8-a): Tune for cortex-a53.
	* config/aarch64/aarch64.md: Do not include aarch64-generic.md.
	* config/aarch64/aarch64.c (aarch64_tune): Initialize to cortexa53.
	(all_cores): Use cortexa53 when tuning for "generic".
	(aarch64_override_options): Fix comment.
	* config/aarch64/aarch64.h (TARGET_CPU_DEFAULT): Set to cortexa53.
	* config/aarch64/aarch64-generic.md: Delete.

From-SVN: r204782