optimize explicit_bzero for size

Avoid saving/restoring the incoming argument by reusing memset return
value.

src/string/explicit_bzero.c

@@ -3,6 +3,6 @@
 
 void explicit_bzero(void *d, size_t n)
 {
-	memset(d, 0, n);
+	d = memset(d, 0, n);
 	__asm__ __volatile__ ("" : : "r"(d) : "memory");
 }