Restore a vital comment nuked in r259016.

sys/sparc64/sparc64/machdep.c

@@ -553,6 +553,13 @@ sparc64_init(caddr_t mdp, u_long o1, u_long o2, u_long o3, ofw_vec_t *vec)
 	 * trigger a fatal reset error or worse things further down the road.
 	 * XXX it should be possible to use this solely instead of writing
 	 * %tba in cpu_setregs().  Doing so causes a hang however.
+	 *
+	 * NB: the low-level console drivers require a working DELAY() and
+	 * some compiler optimizations may cause the curthread accesses of
+	 * mutex(9) to be factored out even if the latter aren't actually
+	 * called.  Both of these require PCPU_REG to be set.  However, we
+	 * can't set PCPU_REG without also taking over the trap table or the
+	 * firmware will overwrite it.
 	 */
 	sun4u_set_traptable(tl0_base);