Make CloudABI work on i386.

Copy over amd64's cloudabi64_sysvec.c into i386 and tailor it to work.
Again, we use a system call convention similar to FreeBSD, except that
there is no support for indirect system calls (%eax == 0).

Where i386 differs from amd64 is that we have to store thread/process
entry arguments on the stack instead of using registers. We also have to
put an extra pointer on the stack for TLS (for GSBASE). Place that
pointer in the empty slot that is normally used to hold return
addresses. That seems to keep the code simple.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D7590