Revert "quickjs: template top level should assign to CFLAGS, not add."

This reverts commit c202cd841b133c1f558767a505988be66195d8b4.

- xbps-src should clean CFLAGS before parsing templates, so adding would
  always be safe
- avoids human error if someone adds CFLAGS above or below this block
  and forgets to change CFLAGS= to CFLAGS+=

srcpkgs/quickjs/template

@@ -21,7 +21,7 @@ fi
 
 if [ "$XBPS_TARGET_NO_ATOMIC8" ]; then
 	makedepends+=" libatomic-devel"
-	CFLAGS="-DLINK_ATOMIC"
+	CFLAGS+=" -DLINK_ATOMIC"
 	export EXTRA_LIBS="-latomic"
 fi