[PATCH] fix typo in libstdc++ configure script

[Paolo Carlini]

Bob Wilson wrote:

> The libstdc++ configure script checks if $atomicity_dir is set to 
> cpu/generic/atomic_mutex, but that directory does not exist.  The 
> correct name is atomicity_mutex, and that is what 
> GLIBCXX_ENABLE_ATOMIC_BUILTINS uses for $atomicity_dir when the 
> builtins aren't available and when there is no cpu-specific atomicity.h.
>
> I noticed this because I stopped seeing the warning about "No native 
> atomic operations are provided for this platform" for Xtensa builds.  
> I verified that with the patch, I am once again seeing that warning.
>
> OK to commit this?

Yes, it's ok, thanks a lot.

Paolo.