Add missing read_buf stub for x86_64-unknown-l4re-uclibc
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.
f943c53c59