Enable mbedTLS dynamic memory allocation in user mode on Windows (#1373)

* change user mode openthread to enable dynamic memory allocation in mbedtls

* complete merge by adding OPENTHREAD_MULTIPLE_INSTANCE to last file

* remove some extraneous changes that got pulled in accidentally

* fix spacing

* fix crash in existing tests due to this change

* fix x86 build

tests/unit/test_platform.cpp

@@ -81,6 +81,18 @@ bool sDiagMode = false;
 
 extern "C" {
 
+#ifdef OPENTHREAD_MULTIPLE_INSTANCE
+    void *otPlatCAlloc(size_t aNum, size_t aSize)
+    {
+        return calloc(aNum, aSize);
+    }
+
+    void otPlatFree(void *aPtr)
+    {
+        free(aPtr);
+    }
+#endif
+
     void otSignalTaskletPending(otInstance *)
     {
     }