[AArch64] Initial -mcpu=ares tuning

[AArch64] Initial -mcpu=ares tuning

This patch adds a tuning struct for the Arm Ares CPU and uses it for -m{cpu,tune}=ares.
The tunings are an initial attempt and may be improved upon in the future, but they serve
as a decent starting point for GCC 9.

With this I see a 1.3% improvement on SPEC2006 int and 0.3% on SPEC2006 fp with -mcpu=ares.
On SPEC2017 I see a 0.6% improvement in intrate and changes in the noise for fprate.

        * config/aarch64/aarch64.c (ares_tunings): Define.
        * config/aarch64/aarch64-cores.def (ares): Use the above.

From-SVN: r268015