tools compiler.h: Add OPTIMIZER_HIDE_VAR()

Port over the definition of OPTIMIZER_HIDE_VAR() so we can use it in
kselftests.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230728-arm64-signal-memcpy-fix-v4-2-0c1290db5d46@kernel.org


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