Fix the `nop` opcode on some MIPS-based Loongson CPUs.

After much trouble building Mono in Debian/MIPS, @directhex
narrowed it down to this issue:

https://sourceware.org/ml/binutils/2009-11/msg00387.html

So since some of the 2E and 2F versions of the Loongson CPUs
break with a regular `sll zero, zero, 0` we need to issue an
`or at, at, 0`. This makes sure we don't randomly deadlock or
blow up when the CPU is under heavy load.

Yes, really.