[Patch libstdc++] Rewrite cpu/generic/atomic_word.h

[David Edelsohn]

I bootstrapped this on powerpc-ibm-aix7.1.0.0 and my colleagues
bootstrapped this on powerpc64-linux and powerpc64le-linux.

It works and produces reasonable instruction sequences.

We can iterate on the syntax, but the core concept seems to work correctly.

Thanks, David