linux: strip installed modules

The installed kernel modules should have useless build cruft stripped out
of them.  On my system, a default build went from a very unreasonable 30MB
to a normal 3MB (on disk) and from 14MB to 3MB when compressed due to the
info stored in the kernel modules alone.

[Peter: don't hardcode binutils strip specific options]
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>