mips o32: provide 64-bit atomics

[kevans]

Summary

This marks mips o32 actually no longer functional in SMP configurations, and CI20/JZ4780 will need to be retired or fixed.

__with_interrupts_disabled style (and in-fact most of the implementations) happily stolen from arm v4 atomics to make auditing correctness simple.

Test Plan

Test booted D22827 all the way to userland, seems functional.

[kevans]

Instead going to opt for reusing D22976, which will remove the need to kill o32 SMP for a little bit.