[RFC liburing for-2.3] Kill src/syscall.c

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

Hi Jens,

I haven't written any patch, but I want to get your comments first.
If you think it's a good idea, I will wire up a patch.

## Background story:

Since we support nolibc, the file src/syscall.c is no longer used by
any liburing internal functions. It's only used by `test/`.

Other architectures that still depend on libc are now using:

     src/generic/syscall.h

*not*:

     src/syscall.c

I should have killed src/syscall.c and fixed the tests earlier when
adding nolibc support, unfortunately, I didn't. I was mistaken that
if we killed src/syscall.c, it might break user. But no, no user uses
it.

We don't actually intend to export __sys_io_uring* functions to user.
This situation is reflected in liburing.map and liburing.h which don't
have those functions.

## Proposal:

Let's just kill src/syscall.c and use __sys_io_uring* or
____sys_io_uring* only.


## Two choices:

1. Keep using ____sys_io_uring* functions and delete __sys_io_uring*
    functions.

2. Back to use __sys_io_uring* functions and delete ____sys_io_uring*.

Both choices need to fix tests. I can do that. I personally like the
second choice since we don't have many underscores.

What do you think?

-- 
Ammar Faizi