Support get return value for SGX os_printf/os_vprintf (#1387)

Fix the issue reported in #1359, change the implementation of
os_printf/os_vprintf for Intel SGX to get the actual bytes written.

product-mini/platforms/linux-sgx/enclave-sample/App/App.cpp

@@ -40,10 +40,10 @@ pal_get_enclave_id(void)
     return g_eid;
 }
 
-void
+int
 ocall_print(const char *str)
 {
-    printf("%s", str);
+    return printf("%s", str);
 }
 
 static char *