Restore default behaviour of signal handling

Arm's new security features (PAC, BTI, MTE) often crash the process with
an appropriate signal in the event of a security problem. When writing
tests to verify this behaviour, it is crucial to ensure that tests exit
with exit codes that reflect the signal.

For Linux this patch removes the explicit termination of the process via
exit(1) and re-raises the signal to invoke the default signal handler
and ensure proper (default) handling of the signal, including exiting
with correct exit code.

On other posix compliant operating systems the signal handler still
terminates the process with an exit code of 1.

Bug: 1137393, 1145581
Change-Id: I0260f8bbccafe38860cef2dbc696701b57fc155c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2777558


Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Cr-Commit-Position: refs/heads/master@{#868466}