[kernel][scheduler] Add plumbing for PI join on fair targets

This adds the plumbing to allow us to enable fair-to-fair inheritance
via channel calls in the future.

Bug: 42182908
Change-Id: I588641b002364450ca885f596f2870b54939fc38
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1499777
Fuchsia-Auto-Submit: Abdulla Kamar <abdulla@google.com>
Commit-Queue: Abdulla Kamar <abdulla@google.com>
Reviewed-by: Corey Tabaka <eieio@google.com>
5 files changed