Make thread creation work for CloudABI processes.

Summary:
Remove the stub system call that was put in place during the system call
import and replace it by a target-dependent version stored in sys/amd64.
Initialize the thread in a way similar to cpu_set_upcall_kse(). We
provide the entry point with two arguments: the thread ID and the
argument pointer.

Test Plan:
Thread creation still seems to work, both for FreeBSD and CloudABI
binaries.

Reviewers: dchagin, mjg, kib

Reviewed By: kib

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D3110