kselftest/arm64: Allow signal tests to trigger from a function

Currently we have the facility to specify custom code to trigger a signal
but none of the tests use it and for some reason the framework requires us
to also specify a signal to send as a trigger in order to make use of a
custom trigger. This doesn't seem to make much sense, instead allow the
use of a custom trigger function without specifying a signal to inject.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211210184133.320748-6-broonie@kernel.org


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