Fixes Android build by using C++ math header instead of C one.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 37827)
+++ runtime/vm/object.cc	(working copy)
@@ -15686,10 +15686,10 @@
 
 
 const char* Double::ToCString() const {
-  if (isnan(value())) {
+  if (std::isnan(value())) {
     return "NaN";
   }
-  if (isinf(value())) {
+  if (std::isinf(value())) {
     return value() < 0 ? "-Infinity" : "Infinity";
   }
   const int kBufferSize = 128;