DPC++ daily 2022-09-27

[SYCL] Enable builds on macOS host (#6706)

This patch contains minimal changes to make compiler toolchain builds
succeed on macOS:
- Add stubs for platform-dependent functions
- Disable level zero plugin for macOS exclusively
- Fix platform-dependent library names
- Remove get_device_count_by_type tool (which is unused, and rather than
fixing it, removal is much simpler)
- Add minimal CI in post-commit to prevent build breakages in future

The above changes are far from fully working SYCL on Darwin targets,
this patch only fixes toolchain builds, and does not attempt to make the
toolchain itself functional.

Fixes #258
Fixes #982