Build(deps): Bump third_party/ibex from 87dfd07 to 8f4c75c

Bumps [third_party/ibex](https://github.com/lowRISC/ibex) from `87dfd07` to `8f4c75c`.
- [Commits](lowRISC/ibex@87dfd07...8f4c75c)

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

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