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>

os/utils.c

@@ -1352,6 +1352,12 @@ OsAbort(void)
 {
 #ifndef __APPLE__
     OsBlockSignals();
+#endif
+#if !defined(WIN32) || defined(__CYGWIN__)
+    /* abort() raises SIGABRT, so we have to stop handling that to prevent
+     * recursion
+     */
+    OsSignal(SIGABRT, SIG_DFL);
 #endif
     abort();
 }