sparc: Use atomic compiler builtins on sparc

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>