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/open.cpp b/libc/bionic/open.cpp
index bd832c0..a6d8086 100644
--- a/libc/bionic/open.cpp
+++ b/libc/bionic/open.cpp
@@ -54,7 +54,7 @@
   if ((flags & O_CREAT) != 0) {
     va_list args;
     va_start(args, flags);
-    mode = (mode_t) va_arg(args, int);
+    mode = static_cast<mode_t>(va_arg(args, int));
     va_end(args);
   }
 
@@ -76,7 +76,7 @@
   if ((flags & O_CREAT) != 0) {
     va_list args;
     va_start(args, flags);
-    mode = (mode_t) va_arg(args, int);
+    mode = static_cast<mode_t>(va_arg(args, int));
     va_end(args);
   }