xen-detect: Avoid dumping core

xen-detect: Avoid dumping core

F12 introduces a tool to automatically report bugs when there are core
dumps.  Since xen-detect relies on fork+waitpid in order to trap a
SIGILL from a child, every time someone runs xen-detect on a bare
metal kernel a bug is reported into Red Hat's Bugzilla. :-)

However, even without this contingent need, leaving core dumps around
is not nice.  So this patch just traps SIGILL using
signal/sentjmp/longjmp, without the need to fork.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>