Implement 64-bit atomic functions on 32-bit mips.

[imp]

Summary

We do this by disabling interrupts, doing the 64-bit op, then re-enabling interrupts. This works only in the kernel, and breaks the JZ4280 port because nobody has written the mips32r6 64-bit atomic ops yet (MP environments can't use the interrupt trick, and the necessary instructions / hardware is only on processors that implement mips32r6 or later).