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

[marvin-hansen]

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 Fix rust_doc_test linking with native libraries #4080 loop to the cc_toolchain runtime libs,
  mirroring the crate-type split in collect_inputs. Guarded on cc_toolchain
  being present (Add support for optional cc_toolchain #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.