[AArch64] Implement -m{cpu,tune,arch}=native using only /proc/cpuinfo

[AArch64] Implement -m{cpu,tune,arch}=native using only /proc/cpuinfo

	* config.host (case ${host}): Add aarch64*-*-linux case.
	* config/aarch64/aarch64-cores.def: Add IMPLEMENTER_ID and PART_NUMBER
	fields to all the cores.
	* config/aarch64/aarch64-elf.h (DRIVER_SELF_SPECS):
	Add MCPU_MTUNE_NATIVE_SPECS.
	* config/aarch64/aarch64-option-extensions.def: Add FEATURE_STRING
	field to all extensions.
	* config/aarch64/aarch64-opts.h: Adjust definition of AARCH64_CORE.
	* config/aarch64/aarch64.c: Adjust definition of AARCH64_CORE.
	Adjust definition of AARCH64_OPT_EXTENSION.
	* config/aarch64/aarch64.h: Adjust definition of AARCH64_CORE.
	(MCPU_MTUNE_NATIVE_SPECS): Define.
	* config/aarch64/driver-aarch64.c: New file.
	* config/aarch64/x-arch64: New file.
	* doc/invoke.texi (AArch64 Options): Document native value for -mcpu,
	-mtune and -march.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@222415 138bc75d-0d04-0410-961f-82ee72b054a4