luajit: only available on x86(-64) hosts

luajit: only available on x86(-64) hosts

The -m32 compiler flag is used for 32bit builds and host-luajit has
limited architecture support. Building for a 32-bit target on a 32-bit
host should always work, but we haven't tested that and it's very
unlikely that someone needs it. So just limit to x86(-64) hosts.

Fixes:
http://autobuild.buildroot.net/results/5f5b5edb058efe976c003678e21bcc28a87cc828/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[Arnout: clarify that it might work on 32-bit hosts for a 32-bit target]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>