kselftest/arm64: Test signal handler state modification in fp-stress

Currently in fp-stress we test signal delivery to the test threads by
sending SIGUSR2 which simply counts how many signals are delivered. The
test programs now also all have a SIGUSR1 handler which for the threads
doing userspace testing additionally modifies the floating point register
state in the signal handler, verifying that when we return the saved
register state is restored from the signal context as expected. Switch over
to triggering that to validate that we are restoring as expected.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241107-arm64-fp-stress-irritator-v2-6-c4b9622e36ee@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>