build(deps): bump libm from 0.2.7 to 0.2.8 in /hermit-builtins

Bumps [libm](https://github.com/rust-lang/libm) from 0.2.7 to 0.2.8.
- [Changelog](https://github.com/rust-lang/libm/blob/master/CHANGELOG.md)
- [Commits](rust-lang/libm@0.2.7...0.2.8)

---
updated-dependencies:
- dependency-name: libm
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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