Don't override signal handlers installed by c-stack.

* src/m4.c (main): Reorder installation of signal handlers, so we
don't trash a SIGBUS handler installed by c-stack.  Also provide a
SIGSEGV fallback, for platforms where c-stack combined with
libsigsegv uses something lower level than SIGSEGV.

Signed-off-by: Eric Blake <ebb9@byu.net>