toolchain: xtensa: Define __sync_synchronize

This builtin gcc function is not available in xcc compiler.

Adding a memory compiler barrier as it is done in compiler_barrier.
compiler_barrier() and __sync_synchronize() are not the same, the
former is a sw barrier while the latter can be a hw barrier
like (mfence/sfence) in X86.

I didn't find anything equivalent for xtensa so just implementing a
SW barrier.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>