This patch removes the arch-specific atomic instruction, relying on
compiler builtins. The __sparc32_atomic_locks support is removed
and a configure check is added to check if compiler uses libatomic
to implement CAS.
It also removes the sparc specific sem_* and pthread_barrier_*
implementations. It in turn allows buidling against a LEON3/LEON4
sparcv8 target, although it will still be incompatible with generic
sparcv9.
Checked on sparcv9-linux-gnu and sparc64-linux-gnu. I also checked
with build against sparcv8-linux-gnu with -mcpu=leon3.
Tested-by: Andreas Larsson <andreas@gaisler.com>
