thread: Add a return value to cpu_set_upcall()

[markj]

Summary

Some implementations copy data to userspace, an operation which can in
principle fail. In preparation for adding a __result_use_check
annotation to copyin() and related functions, let implementations of
cpu_set_upcall() return an error, and check for errors when copying data
to user memory.

[kib]

Might be a worth idea would be to split UVA accessing parts into a separate upcall. Not least because it is really x86-specific.

[markj]

In D43100#982582, @kib wrote:

Might be a worth idea would be to split UVA accessing parts into a separate upcall. Not least because it is really x86-specific.

I don't have much of an opinion on this. Note that the powerpc64 implementation needs to be able to return errors from copyin(). I fixed it in a separate diff.