VLCKit: don't assume x86_64 is the default arch

When referring to the main_build_dir don't assume that the x86_64
subdirectory is a part of the ARCHS that we are building. Grab the
first arch in the list of ARCHS and use that as the main_build_dir.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Pre-Compile.sh

@@ -28,7 +28,7 @@ if test "${ACTION}" = "release-makefile"; then
     main_build_dir="${VLC_BUILD_DIR}"
 else
     use_archs="yes"
-    main_build_dir="${VLC_BUILD_DIR}/x86_64"
+    main_build_dir="${VLC_BUILD_DIR}/${ARCHS%% *}"
     echo "Building for $ARCHS"
 fi