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 <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230728-arm64-signal-memcpy-fix-v4-3-0c1290db5d46@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

Author: broonie

tools/include/linux/compiler.h

@@ -42,6 +42,18 @@
 # 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