[RFC PATCH] Optimized __cxa_guard_{acquire,release,abort} for Linux

[Benjamin Kosnik]

> The patch below doesn't include the configury bits yet (they might be
> very similar to enable_linux_futex stuff in libgomp configury).
> Would this be acceptable with the configury bits in even for 4.3
> (could be e.g. only enabled if explicitly requested with
> --enable-linux-futex there), or just 4.4+

I would rather put this in 4.3, especially considering the new work in
this area from 4.2 hasn't been released yet. Then, we won't be changing
it and changing it again.

As for the configure bits, I suggest using the same bits (exactly) for
both libgomp and libstdc++, to avoid yet more twisty borrowing between
these two runtimes. 

;)

Which means you might as well move it toplevel and avoid the
duplication if possible. Thoughts?

-benjamin