In revision 05c3fee13eb3, openbsd/386's tfork implementation was
accidentally changed in one instruction from using the "params"
parameter to using the "psize" parameter.

While here, OpenBSD's __tfork system call returns a pid_t which is an
int32 on all OpenBSD architectures, so change runtime.tfork's return
type from int64 to int32 and update the assembly implementations
accordingly.

LGTM=iant
R=rsc, iant
CC=golang-codereviews, jsing
https://golang.org/cl/133190043