alpha: fix syscall entry in !AUDUT_SYSCALL case

We only want to take the slow path if SYSCALL_TRACE or SYSCALL_AUDIT is
set; on !AUDIT_SYSCALL configs the current tree hits it whenever _any_
thread flag (including NEED_RESCHED, NOTIFY_SIGNAL, etc.) happens to
be set.

Fixes: a9302e84 "alpha: Enable system-call auditing support"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>