Signed-off-by: onur-ozkan <work@onurozkan.dev>

- UnsafeCell: mention the term "data race", and reference the data race definition
- atomic: failing RMWs are just reads, reorder and reword docs

…in line with the test directory

r? jieyouxu

…=Mark-Simulacrum

atomics: allow atomic and non-atomic reads to race

We currently define our atomics in terms of C++ `atomic_ref`. That has the unfortunate side-effect of making it UB for an atomic and a non-atomic read to race (concretely, [this code](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d1a743774e60923db33def7fe314d754) has UB). There's really no good reason for this, all the academic models of the C++ memory model I am aware of allow this -- C++ just disallows this because of their insistence on an "object model" with typed memory, where `atomic_ref` temporarily creates an "atomic object" that may not be accesses via regular non-atomic operations.

So instead of tying our operations to `atomic_ref`, let us tie them directly to the underlying C++ memory model. I am not sure what is the best way to phrase this, so here's a first attempt.

We also carve out an exception from the "no mixed-size atomic accesses" rule to permit mixed-size atomic reads -- given that we permit mixed-size non-atomic reads, it seems odd that this would be disallowed for atomic reads. However, when an atomic write races with any other atomic operation, they must use the same size.

With this change, it is finally the case that every non-atomic access can be replaced by an atomic access without introducing UB.

Cc `@rust-lang/opsem` `@chorman0773` `@m-ou-se` `@WaffleLapkin` `@Amanieu`

Fixes rust-lang/unsafe-code-guidelines#483

…andling, r=Kobzol

simplify LLVM submodule handling

Fixes rust-lang#130906.

Only add an automatic SONAME for Rust dylibs

rust-lang#126094 added an automatic relative `SONAME` to all dynamic libraries, but it was really only needed for Rust `--crate-type="dylib"`. In Fedora, it was a surprise to see `SONAME` on `"cdylib"` libraries like Python modules, especially because that generates an undesirable RPM `Provides`. We can instead add a `SONAME` just for Rust dylibs by passing the crate-type argument farther.

Ref: https://bugzilla.redhat.com/show_bug.cgi?id=2314879

compiletest: rename "runtest/crash.rs" to "runtest/crashes.rs" to be in line with the test directory

r? jieyouxu

remove couple redundant clones