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

runtime/vm/assembler_mips_test.cc

Index: runtime/vm/assembler_mips_test.cc
===================================================================
--- runtime/vm/assembler_mips_test.cc	(revision 37827)
+++ runtime/vm/assembler_mips_test.cc	(working copy)
@@ -1639,7 +1639,7 @@
   typedef double (*SimpleCode)() DART_UNUSED;
   EXPECT(test != NULL);
   double res = EXECUTE_TEST_CODE_DOUBLE(SimpleCode, test->entry());
-  EXPECT_EQ(isnan(res), true);
+  EXPECT_EQ(std::isnan(res), true);
 }
 
 
@@ -1657,7 +1657,7 @@
   typedef double (*SimpleCode)() DART_UNUSED;
   EXPECT(test != NULL);
   double res = EXECUTE_TEST_CODE_DOUBLE(SimpleCode, test->entry());
-  EXPECT_EQ(isfinite(res), false);
+  EXPECT_EQ(std::isfinite(res), false);
 }