[AArch64] Add -mcpu/-mtune support for Arm Ares

[AArch64] Add -mcpu/-mtune support for Arm Ares

This adds support for the Arm Ares CPU for AArch64.
It implements the Armv8.2-A architecture with the optional features
of statistical profiling, dot product and FP16 on by default.

Note: Ares is a codename to enable early adopters and in time
we will add the final product name once it's announced.

	* config/aarch64/aarch64-cores.def (ares): Define.
	* config/aarch64/aarch64-tune.md: Regenerate.
	* doc/invoke.texi (AArch64 Options): Document ares value for mtune.

From-SVN: r265880