Automatic Rustup

needs a libc version bump

readdir_r shim: assume FreeBSD v12+

Blocked on rust-lang/libc#3723 being released and propagating to std.

It'd be useful to be able to verify code implementing SHA256 using SIMD
since such code is a bit more complicated and at some points requires
use of pointers. Until now `miri` didn't support x86 SHA256 intrinsics.
This commit implements them.

Implement SHA256 SIMD intrinsics on x86

Disclaimer: this is my first contribution to `miri`'s code. It's quite possible I'm missing something. This code works but may not be the cleanest/best possible.

It'd be useful to be able to verify code implementing SHA256 using SIMD since such code is a bit more complicated and at some points requires use of pointers. Until now `miri` didn't support x86 SHA256 intrinsics. This commit implements them.

borrow tracking: add a test for zero-sized protectors

epoll test: avoid some subtly dangling pointers

Turns out `let data = MaybeUninit::<u64>::uninit().as_ptr();` is a dangling pointer, the memory gets freed at the end of that line. For these cases we don't care as we don't actually access the pointer, but let's not do such subtle things.

…r=RalfJung

Make Tree Borrows Provenance GC no longer produce stack overflows

Most functions operating on Tree Borrows' trees are carefully written to not cause stack overflows due to too much recursion. The one exception is [`Tree::keep_only_needed`](https://github.com/rust-lang/miri/blob/94f5588fafcc7d59fce60ca8f7af0208e6f618d4/src/borrow_tracker/tree_borrows/tree.rs#L724), which just uses regular recursion.
This function is part of the provenance GC, so it is called regularly for every allocation in the program.

Tests show that this is a problem in practice. For example, the test `fill::horizontal_line` in crate `tiny-skia` (version 0.11.4) is such a test.

This PR changes this, this test no now longer crashes. Instead, it succeeds (after a _long_ time).