Skip 32-bit Linux build on x86_64 for dir_build without -p

And build_and_run doesn't set -p, so build_and_run will now skip
unnecessary 32-bit builds

app/scripts/dir_build

@@ -54,6 +54,11 @@ if [[ -z $PLATFORM ]]; then
 		PLATFORM="m"
 	elif [ "`uname`" = "Linux" ]; then
 		PLATFORM="l"
+		
+		# If platform not given explicitly, skip 32-bit build if 64-bit system
+		if [ "$(arch)" = "x86_64" ]; then
+			export SKIP_32=1
+		fi
 	elif [ "`uname -o 2> /dev/null`" = "Cygwin" ]; then
 		PLATFORM="w"
 	fi