[AArch64 2/3 big.LITTLE] Allow tuning parameters without unique tuning targets.

[AArch64 2/3 big.LITTLE] Allow tuning parameters without unique tuning targets.
gcc/

	* config/aarch64/aarch64-cores.def: Add new column for
	SCHEDULER_IDENT.
	* config/aarch64/aarch64-opts.h (AARCH64_CORE): Handle
	SCHEDULER_IDENT.
	* config/aarch64/aarch64.c (AARCH64_CORE): Handle
	SCHEDULER_IDENT.
	(aarch64_parse_cpu): mcpu implies a default value for mtune.
	* config/aarch64/aarch64.h (AARCH64_CORE): Handle
	SCHEDULER_IDENT.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@206099 138bc75d-0d04-0410-961f-82ee72b054a4