Weaken release sequences to match the C++20 memory model
See [Weaken Release Sequences](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0982r1.html), since the exception for relaxed writes on the same thread as a release write not blocking release sequences was removed in the C++20 memory model compared to the C11 memory model the paper was based on. The implementation can be updated and simplified to match this. [Rust is currently specified to use the C++20 memory model](https://doc.rust-lang.org/std/sync/atomic/index.html).
remove miri-rustc-tests
`@oli-obk` I am not sure if anyone used this program in the last few years. There are no instructions for how to use it. So I think keeping it working is just a waste of time. It now fails to build:
```
error[E0046]: not all trait items implemented, missing: `visit_foreign_item`
--> src/bin/miri-rustc-tests.rs:36:17
|
36 | impl<'tcx, 'hir> itemlikevisit::ItemLikeVisitor<'hir> for Visitor<'tcx> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `visit_foreign_item` in implementation
```
So I propose instead of me trying to figure out how to fix this, we just drop this ancient code.
Add simple data-race detector
Partially fixes data-race detection, see #1372, based on Dynamic Race Detection for C++11
- This does not explore weak memory behaviour, only exploring one sequentially consistent ordering.
- Data-race detection is only enabled after the first thread is created, so should have minimal overhead for non-concurrent execution.
- ~~Does not attempt to re-use thread id's so creating and joining threads lots of time in an execution will result in the vector clocks growing in size and slowing down program execution~~ It does now
std now looks up `getrandom` and `statx` with `dlsym` before attempting
to use `syscall(SYS_.., ..)`.
It also now passes all arguments as a machine-sized word, instead of
their original types.
Improve contributing instructions
This builds on the changes from https://github.com/rust-lang/miri/pull/1612 and should not be merged before.
- Fix incorrect comment
- Recommend `x.py setup` over manually editing config.toml
- Link to rustc-dev-guide
CONTRIBUTING.md: Use `build --stage 1` instead of `build --stage 2`
It seems to work fine with `--stage 1` and it should be faster.
Cc #1610
Cc `@jyn514`
