The TLB refill and XTLB refill exceptions use distinct exception vectors

The TLB refill and XTLB refill exceptions use distinct exception vectors
on Loongson 3A. Consequently, the kernel has to set up both vectors to
prevent panics with virtual address references.

On Loongson 2F, it is enough to set up the TLB refill vector because
the XTLB refill exception uses the same vector address.

ok miod@