[SPARK-10001][CORE] Don't short-circuit actions in signal handlers

## What changes were proposed in this pull request?
The current signal handlers have a subtle bug that stops evaluating registered actions as soon as one of them returns true, this is because `forall` is short-circuited.
This PR adds a strict mapping stage before evaluating returned result.
There are no known occurrences of the bug and this is a preemptive fix.

## How was this patch tested?
As with the original introduction of signal handlers, this was tested manually (unit testing with signals is not straightforward).

Author: Jakob Odersky <jakob@odersky.com>

Closes #12745 from jodersky/SPARK-10001-hotfix.