unbreak emergency malloc tests on systems without "good" TLS

Our implementation of emergency malloc slow-path actually depends on
good TLS implementation. So on systems without good TLS (i.e. OSX), we
lied to ourselves that emergency malloc is available, but then failed
tests.

src/tcmalloc.cc

@@ -592,11 +592,7 @@ public:
   }
 
   bool HasEmergencyMalloc() override {
-#if ENABLE_EMERGENCY_MALLOC
-    return true;
-#else
-    return false;
-#endif
+    return kUseEmergencyMalloc;
   }
 
   void WithEmergencyMallocEnabled(FunctionRef<void()> body) override {