sme: Support TPIDR2 signal frame context

sme: Support TPIDR2 signal frame context

The Linux Kernel defines a separate context for the TPIDR2 register in a
signal frame.  Handle this additional context in gdb so this register
gets restored properly when unwinding through signal frames.

The TPIDR2 register is closely related to SME, and is available when SME
support is reported.

This is tested by testcases that are available in a later patch in the series.

Regressions-tested on aarch64-linux Ubuntu 22.04/20.04.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>