compiler: Add fallthrough() helper

https://bugzilla.nasm.us/show_bug.cgi?id=3392465

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

include/compiler.h

@@ -240,6 +240,15 @@ size_t strnlen(const char *s, size_t maxlen);
 # define inline_prototypes
 #endif
 
+/*
+ * Fallthrough handling.
+ */
+#ifdef HAVE_FALLTHROUGH_ATTRIBUTE
+# define fallthrough() __attribute__((fallthrough))
+#else
+# define fallthrough() /* fall through */
+#endif
+
 /*
  * Hints to the compiler that a particular branch of code is more or
  * less likely to be taken.