linux-user/aarch64: Support ZT_MAGIC signal frame record

linux-user/aarch64: Support ZT_MAGIC signal frame record

FEAT_SME2 adds the ZT0 register, whose contents may need to be
preserved and restored on signal handler entry and exit.  This is
done with a new ZT_MAGIC record.  We forgot to implement support for
this in our linux-user code before enabling the SME2p1 emulation,
which meant that a signal handler using SME would corrupt the ZT0
register value, and code that attempted to unwind an exception from
inside a signal handler would not work.

Add the missing record handling.

Fixes: 7b1613a1020d2942 ("target/arm: Enable FEAT_SME2p1 on -cpu max")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250725175510.3864231-4-peter.maydell@linaro.org>