Unify architecture io_uring syscall numbers

Unify architecture io_uring syscall numbers

Only Alpha is the odd one out, apart from that all architectures use
the same system call number. So pull the code out arch headers, and
into the generic arch header.

Fixes: https://github.com/axboe/fio/issues/923
Signed-off-by: Jens Axboe <axboe@kernel.dk>