iOS: Allow ARCHS to be specified at build time for multi-arch builds

Building all architectures of a multi-arch build during Qt development
is in most cases not needed, so we expose a way to limit the archs we
build by passing ARCHS="subset of archs" to make, similar to how you
can pass ARCHS to xcodebuild. If the subset doesn't match any of the
valid architectures for the target, it will fall back to the default
architectures, so it's safe to pass eg. ARCHS="armv7 i386" to make,
even if building for both simulator and device. The variable may also
be exported to the environment for more persistent limits on which
architectures to build.

Change-Id: I47b10bc9d743f0301efff4181d6881ae140d557f
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>