Turn on -Wold-style-cast and fix the errors.

A couple of dodgy cases where we cast away const, but otherwise pretty boring.

Change-Id: Ibc39ebd525377792b5911464be842121c20f03b9
diff --git a/libc/bionic/signal.cpp b/libc/bionic/signal.cpp
index 66d75bd..74a2f65 100644
--- a/libc/bionic/signal.cpp
+++ b/libc/bionic/signal.cpp
@@ -43,7 +43,7 @@
     return SIG_ERR;
   }
 
-  return (sighandler_t) sa.sa_handler;
+  return sa.sa_handler;
 }
 
 sighandler_t signal(int signum, sighandler_t handler) {