fix(rustdoc): link doc tests against the cc_toolchain's runtime libs (#4220)

`rust_doc_test` fails to link on any cc_toolchain that supplies its
C++/unwind
runtime via `static_runtime_lib` (i.e. enables
`static_link_cpp_runtimes`).
This happens when cross compiling with the hermetic-LLVM e.g.:

```
ld.lld: error: undefined symbol: _Unwind_Resume
>>> referenced by alloc.rs:0 ... liballoc-*.rlib
... and the rest of the _Unwind_* family
```

`rust_library` / `rust_binary` / `rust_test` are fine. Only doc tests
fail.

## Evidence

The first commit adds only a test to reproduce the issue by extending
the
fixture from #3741 (which already declares a cc_toolchain with
`static_runtime_lib = ":dummy.a"`); the second commit adds the fix:

| commit |
`//test/unit/cc_toolchain_runtime_lib:runtime_libs_test/doc_test` |
|---|---|
| `test(rustdoc): reproduce ...` | **FAILED** |
| `fix(rustdoc): link doc tests ...` | **PASSED** |

The failing action's argv shows the search path arriving while the
library
reference never does:

```
"-Lnative=test/unit/cc_toolchain_runtime_lib",   <- present
...                                              <- no -Clink-arg=-ldummy
```

## Cause

`add_native_link_flags` emits the runtime libs' `-Lnative=`
unconditionally but
gates the matching `-lstatic=` behind `include_link_flags`, which
rustdoc sets
to False (#2467). #4080 added a compensating `-Clink-arg=-l` loop for
doc tests,
but this does not work for a `static_runtime_lib`.

## Alternatives  

Injecting the flag from outside doesn't work. The archive is already in
the
doc test's runfiles (via #3741), so `-Clink-arg=<its short_path>` does
fix doc
tests. However, `toolchain.extra_rustc_flags` applies to every rustc
invocation,
and normal `rust_test` binaries link in the execroot, where that
runfiles-relative path doesn't exist: `clang++: error: no such file or
directory`. Doc tests and normal links need *different paths to the same
file*,
No single toolchain-level flag satisfies both. Per-target
`rustdoc_flags`
does work, but only by hardcoding the canonical repo name and the
runtime lib's
internal layout in every `rust_doc_test`. However, this is
unmaintainable in any major project.

## Changes

- `rustdoc.bzl` — extend the #4080 loop to the cc_toolchain runtime
libs,
mirroring the crate-type split in `collect_inputs`. Guarded on
`cc_toolchain`
  being present (#3665).
- `rustdoc_test.bzl` — add those libs' root to `--strip_substring`.
Required:
  they're built in a different configuration than the crate outputs, so
otherwise the `-Lnative=` path stays an execroot path that doesn't exist
in
the runfiles tree the doc test runs from. Only non-empty roots are added
—
source-file runtime libs need no stripping, and an empty root would emit
  `--strip_substring=/`, which `rustdoc_test_writer` applies as a plain
  `str::replace`.

The libs already reach runfiles via `ctx.runfiles(transitive_files =
action.inputs)`
thanks to #3741.

## Testing

`bazel test -- //... -//test/unit/remap_path_prefix:integration_test` on
macOS:
**569 passed / 35 skipped**, against **568 / 35** before. The new doc
test is
the only target that changed status; every other action was a cache hit,
so
this is a provable no-op where `static_runtime_lib` is empty.
 
The patch has also been verified against a repo that builds with
hermetic-LLVM and all failing doc tests
passed.

---------

Signed-off-by: Marvin Hansen <marvin.hansen@gmail.com>