Use clang-5.0 -Oz

Summary:
Use clang-5.0 -Oz as default NDK compiler for android.
It does 3 good things:
* brings us into compliance with deprecation of gcc for android NDKs since r13
* yields slightly smaller native code, on the order of many-10s-to-small-100s of KB
* slight improvement in startup performance, on the order of 10s of ms

Reviewed By: mzlee

Differential Revision: D6155682

fbshipit-source-id: 2f64e742f4dc44be171274c2ad0a41fb1e7079d7

yoga/Yoga.cpp

@@ -133,6 +133,12 @@ static int YGDefaultLog(const YGConfigRef config,
 #endif
 
 bool YGFloatIsUndefined(const float value) {
+// TODO(gkm): Ugh! Some Android builds (r13b & clang-3.8) fail
+// with the kludge below, so we must tailor it specifically for
+// NDK r15c which has clang-5.0. NDK r16 will make it all better.
+#if __ANDROID__ && __clang_major__ == 5 // TODO(gkm): remove for NDK >= 16
+  using std::isnan;
+#endif
   return isnan(value);
 }