Disable assert code in release builds (define DNDEBUG)

Summary:
Disable assert code in release builds (define DNDEBUG).
This disables assertions in Facebook JNI library - which checks that our references to Yoga JNI objects are correct.
This saves CPU time, since those checks are costly.

Reviewed By: emilsjolander

Differential Revision: D6592023

fbshipit-source-id: eaf869c7990cab3a72ae2bbdc67e709153f5020c

mode/opt

@@ -1,4 +1,4 @@
 --config
-cxx.cxxflags=-O3
+cxx.cxxflags=-O3 -DNDEBUG
 --config
-cxx.cflags=-O3
+cxx.cflags=-O3 -DNDEBUG