os: Handle SIGABRT

Without this, assertion failures can make life hard for users and those
trying to help them.

v2:
* Change commit log wording slightly to "can make life hard", since
  apparently e.g. logind can alleviate that somewhat.
* Set default handler for SIGABRT in
  hw/xfree86/common/xf86Init.c:InstallSignalHandlers() and
  hw/xquartz/quartz.c:QuartzInitOutput() (Eric Anholt)

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
(cherry picked from commit 27a6b9f7)