Improve C11 atomics support:

- Generate atomicrmw operations in most of the cases when it's sensible to do
  so.
- Don't crash in several common cases (and hopefully don't crash in more of
  them).
- Add some better tests.

We now generate significantly better code for things like:
_Atomic(int) x;
...
x++;

On MIPS, this now generates a 4-instruction ll/sc loop, where previously it
generated about 30 instructions in two nested loops.  On x86-64, we generate a
single lock incl, instead of a lock cmpxchgl loop (one instruction instead of
ten).