qt: Disable QtScript on non-supported platforms

There is no support for certain platforms in QtScript right now, so
buildroot upstream suggested to disable the possibility of enabling
QtScript on those platforms as a quick fix for the 2013.11 release:

   http://lists.busybox.net/pipermail/buildroot/2013-November/083157.html

QtScript doesn't build because it uses JavaScriptCore, so we re-use the
existing BR2_PACKAGE_QT_ARCH_SUPPORTS_WEBKIT option which already lists
which platforms are supported by JavaScriptCore in Qt4.

In addition, a proposed fix to add support for MIPS64 has already been
sent to Qt:

   https://bugreports.qt-project.org/browse/QTBUG-35030

Fixes:

   http://autobuild.buildroot.net/results/fd7/fd7e7e53290f235b540ed5f3c784e2584fdb41e0/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>