aarch64: Add support for -mcpu=rigel

This adds support for the NVIDIA Rigel core to the AArch64 backend. The
initial patch does not add any special tuning decisions, and those may come
later.

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

Signed-off-by: Dhruv Chawla <dhruvc@nvidia.com>

gcc/ChangeLog:

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