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/Enclave/Enclave.edl

@@ -19,6 +19,6 @@ enclave {
 
     untrusted {
         /* define OCALLs here. */
-        void ocall_print([in, string]const char* str);
+        int ocall_print([in, string]const char* str);
     };
 };