thread: Add a return value to cpu_set_upcall()

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.

Reviewed by: kib, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43100