[v3] atomics

[Andrew Haley]

David Daney wrote:

>> Do you have any idea as to how many backends this impacts?
> 
> A casual grep of arm, sh, and m68k  indicates that they are probably
> missing _sync*, there may be others too.

To the best of my knowledge this is actually impossible on ARM
because not all versions of the arch have the instructions we need,
and ARM binaries, at least on GNU/Linux systems, are supposed to
work on the different versions.

Linux provides a kernel helper to do the job, so we have a solution
there.

Andrew.