[Clang][AArch64] Add support for -mcpu=grace

This adds Clang command line support for the NVIDIA Grace CPU [1], which
we would like to target with -mcpu=grace.

Grace is based on the Arm Neoverse V2 CPU [1], which is why, in the
driver, we pass neoverse-v2 to the compiler when grace is requested.
Thus, for now, it's an alias to Neoverse V2.

This adds support for grace in user-facing tool Clang, but not yet the
developer tools like opt and llc, so we probably want to follow up on
this.

[1] https://www.nvidia.com/en-us/data-center/grace-cpu/

Differential Revision: https://reviews.llvm.org/D136425