kselftest/arm64: Use shared OPTIMZER_HIDE_VAR() definiton

We had open coded the definition of OPTIMIZER_HIDE_VAR() as a fix but now
that we have the generic tools/include available and that has had a
definition of OPTIMIZER_HIDE_VAR() we can switch to the define.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230728-arm64-signal-memcpy-fix-v4-5-0c1290db5d46@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>