[Pal/Linux-SGX] Force ocall_gettime() to loop on EINTR

SGX PAL-level functions like _DkSystemTimeQuery() assume that
ocall_gettime() does not return EINTR. Thus, this commit forces
this ocall to loop on EINTR.

Pal/src/host/Linux-SGX/enclave_ocalls.c

@@ -972,7 +972,9 @@ int ocall_gettime (unsigned long * microsec)
         return -EPERM;
     }
 
-    retval = sgx_ocall(OCALL_GETTIME, ms);
+    do {
+        retval = sgx_ocall(OCALL_GETTIME, ms);
+    } while(retval == -EINTR);
     if (!retval)
         *microsec = ms->ms_microsec;