Move sched_getaffinity back to Linux
This reverts commit c1a3f8576ea12b0bed68ad3dedf4069ca3d9816f. The shim isn't actually useful for anything, and it is untested on FreeBSD. On Linux it exists solely because std and num_cpus are trying this before falling back to `sysconf`, but on FreeBSD that's not how they work, so there's no reason I can see to have this stub shim on FreeBSD.
interpret, miri: uniform treatments of intrinsics/functions with and without return block
A long time ago we didn't have a `dest: &MPlaceTy<'tcx, Self::Provenance>` for diverging functions, and since `dest` is used so often we special-cased these non-returning intrinsics and functions so that we'd have `dest` available everywhere else. But this has changed a while ago, now only the return block `ret` is optional, and there's a convenient `return_to_block` function for dealing with the `None` case.
So there no longer is any reason to treat diverging intrinsics/functions any different from those that do return.
Use `unchecked_sub` in `split_at`
LLVM currently isn't figuring it out on its own, even in the checked version where it hypothetically could.
Before: <https://rust.godbolt.org/z/PEY38YrKs>
```llvm
bb1: ; preds = %start
%4 = getelementptr inbounds float, ptr %x.0, i64 %n
%5 = sub i64 %x.1, %n
```
After:
```llvm
bb1: ; preds = %start
%4 = getelementptr inbounds float, ptr %x.0, i64 %n
%5 = sub nuw i64 %x.1, %n
```
This is not using the wrapper because there's already a ubcheck covering it, so I don't want this to get a second one once #121571 lands.
---
This is basically the same as #108763, since `split_at` is essentially doing two `get_unchecked`s.
Various improvements to entrypoint code
This moves some code around and adds some documentation comments to make it easier to understand what's going on with the entrypoint logic, which is a bit complicated.
The only change in behavior is consolidating the error messages for unix_sigpipe to make the code slightly simpler.
Improve several `Read` implementations
- `read_to_end` and `read_to_string` for `Cursor`
- Error on OOM in `read_to_string` of `&[u8]` and `VecDeque<u8>`
- Avoid making the slices contiguous in `VecDeque::read_to_string`
- ~`read_exact` and (unstable) `read_buf_exact` for `Take`~
- ~`read_buf` for `UnixStream` and `&UnixStream`~ (moved to #123084)
- `read_to_end` for `ChildStdErr`
sync: better error in invalid synchronization primitive ID
`@devnexen` this should fix the ICE in your PR (but it won't fix the code, it will just report proper UB instead).
