Change new thread's affinity after thread starts, from the same thread, as a workarou...

Change new thread's affinity after thread starts, from the same thread, as a workaround for Snap (#40205)

- Snap's default strict confinement doesn't allow setting the affinity of a different thread, and currently doesn't allow `sched_setaffinity(<nonzeroPid>, ...)`, which pthread implementation calls
- Switched to use sched_setaffinity(0, ...) where appropriate

Fix for https://github.com/dotnet/runtime/issues/1634 in master