util: Make unreachable at least be an assert

Previously, if __builtin_unreachable() was unavailable, the
unreachable macro was defined to do nothing. We do better here, by at
least still making it an assert.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/util/macros.h

@@ -82,7 +82,7 @@ do {                        \
 #endif
 
 #ifndef unreachable
-#define unreachable(str)
+#define unreachable(str) assert(!str)
 #endif
 
 /**