kselftest/arm64: Provide a SIGUSR1 handler in the kernel mode FP stress test

kselftest/arm64: Provide a SIGUSR1 handler in the kernel mode FP stress test

The other stress test programs provide a SIGUSR1 handler which modifies the
live register state in order to validate that signal context is being
restored during signal return. While we can't usefully do this when testing
kernel mode FP usage provide a handler for SIGUSR1 which just counts the
number of signals like we do for SIGUSR2, allowing fp-stress to treat all
the test programs uniformly.

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-v2-5-c4b9622e36ee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>