Using __sync_* builtins within libgcc code

[Paolo Carlini]

Joseph S. Myers wrote:
> I hold that it is a bug that i686-* tools default to -march=i386 instead 
> of -march=i686 (whereas e.g. sparcv9-* tools default to -mcpu=sparcv9, and 
> -mcpu means -march for SPARC).
Seconded.

Paolo.