(dist-scripts) fix $big_stack check

-missing whitespace around the = operator made the statement always
evaluate to true.
-missing quotes made the conditional result in a syntax error when
$big_stack is empty.

dist-scripts/dist-cores.sh

@@ -113,7 +113,7 @@ for f in *_${platform}.${EXT} ; do
    cp -f "$f" ../libretro_${platform}.${EXT}
 
    # Do cleanup if this is a big stack core
-   if [ $big_stack="BIG_STACK=1" ] ; then
+   if [ "$big_stack" = "BIG_STACK=1" ] ; then
       if [ $MAKEFILE_GRIFFIN = "yes" ]; then
          make -C ../ -f Makefile.griffin platform=${platform} clean || exit 1
       elif [ $PLATFORM = "emscripten" ]; then
@@ -178,7 +178,7 @@ for f in *_${platform}.${EXT} ; do
    fi
 
    # Do cleanup if this is a big stack core
-   if [ $big_stack="BIG_STACK=1" ] ; then
+   if [ "$big_stack" = "BIG_STACK=1" ] ; then
       if [ $MAKEFILE_GRIFFIN = "yes" ]; then
          make -C ../ -f Makefile.griffin platform=${platform} clean || exit 1
       elif [ $PLATFORM = "emscripten" ]; then