compiler: Define OPTIMIZER_HIDE_VAR

Part of upstream commit fe8c8a126806 ('crypto: more robust
crypto_memneq'), needed by commit d4c5efdb9777 ('random: add and use
memzero_explicit() for clearing data').

diff --cc include/linux/compiler-gcc.h
Simple merge

diff --cc include/linux/compiler-intel.h
Simple merge

diff --cc include/linux/compiler.h
Simple merge