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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

tools/include/linux/compiler.h

···
 #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 */