Port r212559 to mips.

Do not explicitly enable interrupts in smp_init_secondary() because it
renders any spinlock protected code after that point to run with
interrupts enabled. This is because the processor is executing in the
context of idlethread whose 'md_spinlock_count' is already set to 1.

Instead just let sched_throw() re-enable interrupts when it releases
the spinlock.

The original powerpc commit log for r212559 is available here:
http://svn.freebsd.org/viewvc/base?view=revision&revision=212559

sys/mips/mips/mp_machdep.c

@@ -310,8 +310,6 @@ smp_init_secondary(u_int32_t cpuid)
 	while (smp_started == 0)
 		; /* nothing */
 
-	intr_enable();
-
 	/* Start per-CPU event timers. */
 	cpu_initclocks_ap();