Add missing read_buf stub for x86_64-unknown-l4re-uclibc

[saethlin]

Before this PR, x check library/std --target x86_64-unknown-l4re-uclibc will fail with

error[E0599]: no method named `read_buf` found for struct `Socket` in the current scope
   --> std/src/os/unix/net/stream.rs:598:16
    |
598 |         self.0.read_buf(buf)
    |                ^^^^^^^^
    |
   ::: std/src/sys/pal/unix/l4re.rs:23:5
    |
23  |     pub struct Socket(FileDesc);
    |     ----------------- method `read_buf` not found for this struct
    |
    = help: items from traits can only be used if the trait is implemented and in scope

This target doesn't have a maintainer to cc.

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[Noratrieb]

if it doesn't have a maintainer to cc, maybe it's time for it to go :3 (and if someone complains they can become the maintainer).
when was this added?

[Noratrieb]

in #43639. #112595 was the most recent specific change.

[Noratrieb]

@hargoniX I see you've contributed a fix to the l4re target within the last year. do you still use this target and if yes, would you be interested in becoming a target maintainer (see the target tier policy for what to expect).
if no maintainer is found, the target might be removed in the future.

[Noratrieb]

@bors r+ rollup

[bors]

📌 Commit fcb7d3f has been approved by Noratrieb

It is now in the queue for this repository.

[hargoniX]

@hargoniX I see you've contributed a fix to the l4re target within the last year. do you still use this target and if yes, would you be interested in becoming a target maintainer (see the target tier policy for what to expect). if no maintainer is found, the target might be removed in the future.

Unfortunately not. It was a project for my Bachelor thesis and I've since moved on to other ones. Maybe @humenda is interested or knows somebody that is interested?

[humenda]

I'm unfortunately not working on L4Re anymore. However, I've pinged Kernkonzept, company behind L4Re, and I hope they can appoint a maintainer.

[farao]

Hi, Marius from Kernkonzept here. Yes, I'd like to step in here. How do you become a maintainer for a target? I'm actually currently doing changes to how the linking of rust for l4re is done (without the need of the l4-bender wrapper script which is using meta information from the l4re build) and I will provide a PR soon.

[saethlin]

The target in question doesn't have a documentation page, so the first thing is to add one. You'll be adjusting this line:

rust/src/doc/rustc/src/platform-support.md

Line 389 in 7d40450

`x86_64-unknown-l4re-uclibc` | ? | |

to be a link like the surrounding lines, then actually providing a target for that link in platform-support. I don't think we have an actual template, but starting by copying from a similar target is probably good.

The important thing is that there's a page and it mentions you and your GitHub account so people can get in touch when there's a problem.