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

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

This adds support for the Arm Ares CPU in the arm port.
It implements the Armv8.2-A architecture with the relevant optional
features
of 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-arm.c (arm_cpus): Add ares.
	* doc/c-arm.texi (-mcpu): Document ares value.