target/arm: Implement org.gnu.gdb.aarch64.tls XML feature in gdbstub

target/arm: Implement org.gnu.gdb.aarch64.tls XML feature in gdbstub

GDB expects the TLS registers to be exposed via org.gnu.gdb.aarch64.tls,
which will contain either just "tpidr", or else "tpidr" and "tpidr2".

This will be important for SME in future, because the lazy state
restoration scheme requires GDB to use the TPIDR2 information.
GDB doesn't currently implement that, but we should provide the
register via the XML so that we are ready when future GDB versions
support it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20251017153027.969016-3-peter.maydell@linaro.org