[AArch64] Add support for Arm Cortex-A76

[AArch64] Add support for Arm Cortex-A76

The Cortex-A76 is an Armv8.2-A processor with dotproduct and FP16 support.
It can be paired with the Cortex-A55 and hence the option
-mcpu/-mtune=cortex-a76.cortex-a55 is also introduced.

Bootstrapped and tested on aarch64-none-linux-gnu.

	* config/aarch64/aarch64-cores.def (cortex-a76): New entry.
	(cortex-a76.cortex-a55): Likewise.
	* config/aarch64/aarch64-tune.md: Regenerate.
	* doc/invoke.texi (AArch64 Options): Document cortex-a76 and
	cortex-a76.cortex-a55.

From-SVN: r262186