arm64: add ptrace regsets for ptrauth key management

Add two new ptrace regsets, which can be used to request and change the
pointer authentication keys of a thread. NT_ARM_PACA_KEYS gives access
to the instruction/data address keys, and NT_ARM_PACG_KEYS to the
generic authentication key.

The regsets are only exposed if the kernel is compiled with
CONFIG_CHECKPOINT_RESTORE=y, as the intended use case is checkpointing
and restoring processes that are using pointer authentication. Normally
applications or debuggers should not need to know the keys (and exposing
the keys is a security risk), so the regsets are not exposed by default.

Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>