[AArch64][gas] Add -mcpu support for Arm Ares

[AArch64][gas] Add -mcpu 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/tc-aarch64.c (aarch64_cpus): Add ares.
    * doc/c-aarch64.texi (-mcpu): Document ares value.