tools include: Add some common function attributes

We don't have definitions of __always_unused or __noreturn in the tools
version of compiler.h, add them so we can use them 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 __always_inline	inline __attribute__((always_inline))
 #endif
 
+#ifndef __always_unused
+#define __always_unused __attribute__((__unused__))
+#endif
+
+#ifndef __noreturn
+#define __noreturn __attribute__((__noreturn__))
+#endif
+
+#ifndef unreachable
+#define unreachable() __builtin_unreachable()
+#endif
+
 #ifndef noinline
 #define noinline
 #endif