bench-slope: restore compiler barriers for auto-GHZ check

* tests/bench-slope.c (auto_ghz_bench): Add memory barriers to
AUTO_GHZ_OPERATION macro when barrier is supported by compiler.

Clang is buggy and is optimizing away the auto-GHZ loop even
when 'one' and 'two' variables have been derived from volatile
variables. Restore compiler barrier to ensure that auto-GHZ
calibration is not optimized away.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>