mono: only available for x86/x86-64 host architectures

Fixes:

http://autobuild.buildroot.net/results/3cab7b54390feb5f952407f1e7fa49b9633a5f76/
http://autobuild.buildroot.net/results/ad9fc1f8a598d2c1ee7e6f8884f897f7b8bbb657/



And many others.

Mono needs host-mono, which fails to build on the ppc64le autobuilder.
There is unlikely to be any real use cases of people building mono on !x86,
so only make it available on x86/x86-64 hosts, similar to how it was
recently done for erlang.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>