Teach gdb about the ZA/SSVE state on signal frames and how to restore
the contents of the registers.
There is a new ZA_MAGIC context that the Linux Kernel uses to communicate
the ZA register state to gdb.
The SVE_MAGIC context has also been adjusted to contain a flag indicating
whether it is a SVE or SSVE state.
Regression-tested on aarch64-linux Ubuntu 22.04/20.04.
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
