Include FreeBSD in conditional compile attributes

[awbradle]

Connections
Fixes #8175

Description
Changes from #7076 cause the Vulkan and GLES backends to not build for FreeBSD. I have added FreeBSD to the conditional build attributes introduced in this change.

Testing
Tested by running the examples in the example directory. These are now able to find a GPU adapter and run. Most tests refuse to run prior to making this change due to lack of an adapter.

Squash or Rebase?
Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[cwfitzgerald]

Thanks for handling this! Code looks fine, could you add a CI job to the CI matrix to make sure that freebsd compiles?

[awbradle]

I don't see that GitHub offers a hosted FreeBSD runner. I think there is a more complicated way to use a vm https://github.com/marketplace/actions/freebsd-vm but this looks like it would require more work to set up beyond adding another option to the matrix. Should this be a new issue?

[cwfitzgerald]

Nah, don't have to go that far, just add the target triple to the list https://github.com/gfx-rs/wgpu/blob/trunk/.github/workflows/ci.yml#L75-L183 which will run cargo clippy against that target. We won't test or ensure it links, but at the very least it will compile and there won't be warnings. I'd add it after linux before android, and should be able to do the wgpu-only kind, so we won't need to build/link the C code the tests use.