Make fcntl() a weak symbol so that it can be overridden by thread libraries.

MFC after: 2 days

lib/libc/sys/fcntl.c

@@ -34,8 +34,10 @@ __FBSDID("$FreeBSD$");
 #include <sys/syscall.h>
 #include "libc_private.h"
 
+__weak_reference(__fcntl, fcntl);
+
 int
-fcntl(int fd, int cmd, ...)
+__fcntl(int fd, int cmd, ...)
 {
 	va_list args;
 	long arg;