qt5-qtwebengine: disable aarch64 and armhf (see #1008)

(Not increasing the pkgrel, because we really don't want to build
this when not necessary.)

aports/main/qt5-qtwebengine/APKBUILD

 pkgname=qt5-qtwebengine
 pkgver=5.9.3
 pkgrel=0
-arch="all"
+# Disabled for armhf, aarch64 (see #1008)
+arch="x86_64"
 url='http://qt-project.org/'
 license="GPL3 LGPL3 FDL custom"
 pkgdesc='Provides communication between JS/HTML and Qt apps'