qt5webengine: only available on x86(-64) hosts

qt5webengine: only available on x86(-64) hosts

The -m32 compiler flag is used for 32bit builds and the host tools have
limited architecture support. It may work in some situations (e.g.
on aarch64 host when building a 64-bit target), but it's equally likely
that it breaks and those situations are not tested. So just limit to
x86(-64) hosts.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[Arnout: explain that it may work in some situations]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>