linux: Remove INTERNAL_SYSCALL_DECL

With all Linux ABIs using the expected Linux kABI to indicate
syscalls errors, the INTERNAL_SYSCALL_DECL is an empty declaration
on all ports.

This patch removes the 'err' argument on INTERNAL_SYSCALL* macro
and remove the INTERNAL_SYSCALL_DECL usage.

Checked with a build against all affected ABIs.

sysdeps/unix/sysv/linux/exit-thread.h

@@ -32,7 +32,6 @@ __exit_thread (void)
      of the caller and doing unexpectedly strange things.  */
   while (1)
     {
-      INTERNAL_SYSCALL_DECL (err);
-      INTERNAL_SYSCALL (exit, err, 1, 0);
+      INTERNAL_SYSCALL_CALL (exit, 0);
     }
 }