[aarch64] Add support for -mcpu=grace

[aarch64] Add support for -mcpu=grace

This adds support for the NVIDIA Grace CPU to aarch64.
We reuse the tuning decisions for the Neoverse V2 core, but include a
number of architecture features that are not enabled by default in
-mcpu=neoverse-v2.

This allows Grace users to more simply target the CPU with -mcpu=grace
rather than remembering what extensions to tag on top of
-mcpu=neoverse-v2.

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

	* config/aarch64/aarch64-cores.def (grace): New entry.
	* config/aarch64/aarch64-tune.md: Regenerate.
	* doc/invoke.texi (AArch64 Options): Document the above.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>