build(deps): bump third_party/yosys from 6c8ae44 to a739e21

Bumps [third_party/yosys](https://github.com/YosysHQ/yosys) from `6c8ae44` to `a739e21`.
- [Release notes](https://github.com/YosysHQ/yosys/releases)
- [Commits](YosysHQ/yosys@6c8ae44...a739e21)

---
updated-dependencies:
- dependency-name: third_party/yosys
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>