lguest: clean up example launcher compile flags.

18 months ago 5bbf89fc changed to loading
bzImages directly, and no longer manually ungzipping them, so we no longer
need libz.

Also, -m32 is useful for those on 64-bit platforms (and harmless on
32-bit).

Reported-by: Ron Minnich <rminnich@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>