alpha: Fix up new futex syscall numbers

As per Arnd, Alpha syscalls since time64 are offset by 120, retain
this offset.

Fixes: 9f6c532f ("futex: Add sys_futex_wake()")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/cb4bb8e2-7dfe-4ca4-aa70-060f7b2f8f95@app.fastmail.com