Turn on -Wunused and fix the mistakes it uncovers.

Change-Id: I023d2d8b547fbc21d4124bb7510d42b06a0dc501

libc/bionic/libc_init_static.cpp

@@ -85,7 +85,7 @@ static void apply_gnu_relro() {
 }
 
 __noreturn void __libc_init(void* raw_args,
-                            void (*onexit)(void),
+                            void (*onexit)(void) __unused,
                             int (*slingshot)(int, char**, char**),
                             structors_array_t const * const structors) {
   KernelArgumentBlock args(raw_args);