[rule-based toolchain] Support coverage (gcov)

Bazel handles some tools special in that they can only be specified via `tool_path()`, not `action_config()`. This makes the rule-based toolchains completely unusable if one cared about coverage.

Start by examining the rule-based toolchain handling for tools that must be specified through tool_path() rather than action_config(). Trace how coverage and gcov are configured, then verify that a rule-based toolchain can use gcov successfully for coverage. The issue names no files or tests, so locating the relevant implementation and validation is part of the work.

Written by the indexing model from the issue text.