Add C11 atomic fallbacks for ARM.

Add C11 atomic fallbacks for ARM.

Basically the situation is as follows:

- When using Clang + armv6, we should not need any intrinsics. It should
  support it, even though due to a target misconfiguration it does not.
  We should fix this in Clang.
- When using Clang + noarmv6, provide __atomic_* functions that disable
  interrupts.
- When using GCC + armv6, we can provide __sync_* intrinsics, similar to
  what we did for MIPS. As ARM and MIPS are quite similar, simply base
  this implementation on the one I did for MIPS.
- When using GCC + noarmv6, disable the interrupts, like we do for
  Clang.

This implementation still lacks functions for noarmv6 userspace. To be
done.