bench-slope: update auto-GHZ for alderlake-P

* tests/bench-slope.c (vone): New.
(auto_ghz_bench): Remove memory barrier usage; Generate constant
values from volatile variable.

Alderlake P-core is able of fuse "immediate to register
addition" instructions with other instructions. This feature
broke auto-GHZ detection in bench-slope. Patch forces generation
of "register to register addition" by generating constant values
from volatile source, which prevents compiler from known the
values in constant value variables.

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