On iOS the compiler expects archs like armv6, armv7, armv7s when passed the
-arch flag, or when the ARCHS Xcode variable is set. Instead of mangling
QT_ARCH, which is used other places and assumed to match the values
provided by the arch.test, we use our own variable.
Change-Id: I05e10be8d69dd4d7cbcef04640fef99f1efb253d
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>