Older versions of the Arm architecture lack support for __sync
operations directly in hardware and require calls into appropriate
operating-system hooks. But such hooks obviously don't exist in a
freestanding environment.
Consquently, it is incorrect to assume during configure that such
functions will exist and we need a configure-time check to determine
whether or not these routines will work.
libbacktrace:
* configure.ac: Always check if the compiler supports __sync
operations.
* configure: Regenerated.
