Support CPU masks and affinities for tasks

[remexre]

Right now, the scheduler doesn't support any notion of a task that can only run on some harts (either because the user cares about placement, or because the task will use some ISA extensions that aren't available on all harts), or that prefers to be one some hart (e.g. for performance reasons).

Some smarter data structures might be needed to do this performantly.

[remexre]

Sounds like SpacemiT's next SoC is going to have different vector sizes between the two types of cores, so we'd need to solve this to support that SoC.