fix build.rs invoking RUSTC to do check builds

This makes the Miri driver, when invokved via the RUSTC env var from inside a build script, behave almost entirely like rustc. I had to redo how we propagate sysroot information for this (which is actually back to how we used to do sysroot propagation many years ago).

Fixes https://github.com/rust-lang/miri/issues/2431

ui_test: build dependencies in locked mode unless bless is enabled

This should help ensure that tests are reproducible.

rustup

more tests for ptr_offset_from_unsinged

this requires a change in sysroot handling: miri driver now requires
MIRI_SYSROOT to be set when it is in 'target' mode, rather than relying on
`--sysroot` always being present.

Rustup

attempt to overwrite globally set build.rustc-wrapper

Hopefully fixes https://github.com/rust-lang/miri/issues/2429

cargo-miri: use '--config target.runner' rather than the TARGET_RUNNER env vars

That means we can properly escape spaces in our filename. This should fix https://github.com/rust-lang/miri/issues/2417, hopefully for real this time. We can also specify a "leading command" which makes it much easier to disambiguate runner invocations from rustdoc invocations!

Thanks to `@ehuss` for suggesting this.

Fix outdated docs in sb stack cache

Since `Item` is bitpacked now, the full `Item` is stored in the cache.

Since `Item` is bitpacked now, the full `Item` is stored in the cache.

also clean up sysroot building printing logic a bit

don't dump xargo output onto users of 'cargo miri test'

The xargo invocation prints a lot of details users probably won't care about, so let's hide them (unless the user did `cargo miri setup`, then we still print everything).

add a flag to print a diagnostic when an outdated value is returned from an atomic load

Helps with https://github.com/rust-lang/miri/issues/2313. It can still be annoying to figure out *which* outdated load is the important one in case there are many of them (and the issue contains some ideas for how to help with that situation), but having this flag is better than nothing.

Thanks to `@cbeuw` for the [original patch](https://github.com/cbeuw/miri/commit/64d738cb00dbcfbe9cab29c77717f5af62c83ada) that I based this on.

move num_cpus test from test-cargo-miri to crate test

add more track_caller tests

Suggested by `@EddyB`

support MIRI_HOST_SYSROOT env var for stage 0 builds

Together with a [patch on the rustc side](https://github.com/rust-lang/rust/pull/99599), this makes `./x.py test src/tools/miri --stage 0` work again. :)

r? `@oli-obk`

portable-simd: test bitmasks smaller than a byte

Blocked on https://github.com/rust-lang/portable-simd/pull/267 propagating to the [rustc repo](https://github.com/rust-lang/rust/tree/master/library/portable-simd)

Together with a patch on the rustc side, this makes './x.py test src/tools/miri --stage 0' work again. :)

adjust for symbolic vtables

The Miri side of https://github.com/rust-lang/rust/pull/99420