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

tools/include/linux/compiler.h

@@ -190,4 +190,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 #define ___PASTE(a, b) a##b
 #define __PASTE(a, b) ___PASTE(a, b)
 
+#ifndef OPTIMIZER_HIDE_VAR
+/* Make the optimizer believe the variable can be manipulated arbitrarily. */
+#define OPTIMIZER_HIDE_VAR(var)						\
+	__asm__ ("" : "=r" (var) : "0" (var))
+#endif
+
 #endif /* _TOOLS_LINUX_COMPILER_H */