… r=RalfJung

ptr_offset_unsigned_overflow: extend test

Test that indeed, the signed version works before the unsigned version is UB.

It looks cleaner and makes it consistent with other X86 tests

Use `@only-target` in SSE and SSE2 tests too

It looks cleaner and makes it consistent with other X86 tests.

The huge diffs are mostly indentation changes.

Rustup

This has a larger large "fmt" diff, probably there was a bug rustfmt update. For some reason the automatic `./miri fmt` on CI failed so this PR had to be created by hand -- it is unclear to me why this occurred.

Rustup

try to give the CI job permission to create PRs

See the discussion [here](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/did.20github.20token.20permissions.20change.3F).

`@rust-lang/infra` just a heads-up that we are doing this -- I have no idea if this could cause any issues elsewhere, or if it is important that we limit the permissions here somehow, or so.

… len in read to usize

Refactor fd read/write

This PR passed the responsibility of reading to user supplied buffer and dest place to each implementation of ``FileDescription::read/write/pread/pwrite``.

This is part of rust-lang#3665.