sem-bare.h: Use seL4_WaitWithMRs syscall for mcs

This syscall doesn't need a reply cap to be set on mcs and can work
without an IPC buffer setup.

libsel4sync/include/sync/sem-bare.h

diff --git a/libsel4sync/include/sync/sem-bare.h b/libsel4sync/include/sync/sem-bare.h
index a635fbd..e0d7ef7 100644
--- a/libsel4sync/include/sync/sem-bare.h
+++ b/libsel4sync/include/sync/sem-bare.h
@@ -41,14 +41,18 @@
     if (oldval <= 0) {
 #ifdef CONFIG_ARCH_IA32
 #ifdef CONFIG_KERNEL_RT
-        seL4_RecvWithMRs(ep, NULL, NULL);
+        seL4_WaitWithMRs(ep, NULL, NULL);
 #else
         seL4_RecvWithMRs(ep, NULL, NULL, NULL);
 #endif /* CONFIG_KERNEL_RT */
 #else // all other platforms have 4 mrs
-
+#ifdef CONFIG_KERNEL_RT
+        seL4_WaitWithMRs(ep, NULL, NULL, NULL, NULL, NULL);
+#else
         seL4_RecvWithMRs(ep, NULL, NULL, NULL, NULL, NULL);
 #endif
+
+#endif
         /* Even though we performed an acquire barrier during the atomic
          * decrement we did not actually have the lock yet, so we have
          * to do another one now */