feat(make): implement iOS build (#316)

Part of https://github.com/ooni/probe/issues/1440.

Basically, let us continue to update our build infrastructure so that we can release v3.10.0-beta.

Now, it's the turn of iOS.

MOBILE/gomobile

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+# print the fullpath of the go/gomobile we're using so that it's part
+# of the build logs and we can easily increase our confidence
+# that we are using the right go/gomobile binary.
+
+echo -n "$0: checking for go... "
+go=`command -v go`
+if [ -z $go ]; then
+    echo "not found"
+    exit 1
+fi
+echo "$go"
+
+echo -n "$0: checking for gomobile... "
+gomobile=`command -v gomobile`
+if [ -z $go ]; then
+    echo "not found"
+    exit 1
+fi
+echo "$gomobile"
+
+set -x
+$gomobile "$@"