In particular, this includes a fix to `iai-callgrind` that will allow us
to simplify our benchmark runner.

iai-callgrind now correctly exits with error if regressions were found
[1], so we no longer need to check for regressions manually. Remove this
check and instead exit based on the exit status of the benchmark run.

[1] iai-callgrind/iai-callgrind#337

`binop_common` emits a `SKIP` that is intended to apply only to
`copysign`, but is instead applying to all binary operators. Correct the
general case but leave the currently-failing `maximum_num` tests as a
FIXME, to be resolved separately in [1].

Also simplify skip logic and NaN checking, and add a few more `copysign`
checks.

[1]: #939

compiler-builtins has a symlink to the `libm` source directory so the
two crates can share files but still act as two separate crates. This
causes problems with some sysroot-related tooling, however, since
directory symlinks seem to not be supported.

The reason this was a symlink in the first place is that there isn't an
easy for Cargo to publish two crates that share source (building works
fine but publishing rejects `include`d files from parent directories, as
well as nested package roots). However, after the switch to a subtree,
we no longer need to publish compiler-builtins; this means that we can
eliminate the link and just use `#[path]`.

Similarly, the LICENSE file was symlinked so it could live in the
repository root but be included in the package. This is also removed as
it caused problems with the dist job (error from bootstrap's
`tarball.rs`, "generated a symlink in a tarball").

If we need to publish compiler-builtins again for any reason, it would
be easy to revert these changes in a preprocess step.

The submodule was causing issues in rust-lang/rust, so eliminiate it
here. `build-musl` is also removed from `libm-test`'s default features
so the crate doesn't need to be built by default.

This will be used by `josh` tooling.

Create a crate that handles pulling from and pushing to rust-lang/rust.
This can be invoked with the following:

    $ cargo run -p josh-sync -- rustc-pull
    $ RUSTC_GIT=/path/to/rust/checkout cargo run -p josh-sync -- rustc-push <username>

To prepare for merging from rust-lang/rust, set the version file to:

    df8102fe5f Auto merge of #142002 - onur-ozkan:follow-ups2, r=jieyouxu

…ub.com/rust-lang/rust

Pull recent changes from rust-lang/rust via Josh.

Upstream ref: df8102fe5f24f28a918660b0cd918d7331c3896e
Filtered ref: 3c30d8c

This was introduced before `#[panic_handler]` was stable, but should no
longer be needed. Additionally, we only need it for
`builtins-test-intrinsics`, not as a dependency of `compiler-builtins`.