This commit adds linux/s390x support to the unix package. It is
based on the changes made to the syscall package in
https://golang.org/cl/20961/. It also adds mkpost.go which is
used to cleanup the API generated by cgo -godefs.

The biggest departure that is made with the syscall package is
the use of the -fsigned-char flag to force signed chars. We
couldn't do this in the syscall package because of the need to
maintain compatibility with the gccgo implementation of the syscall
package (gccgo has supported s390x for a longer time than the Go
toolchain). The unix package does not have this constraint.

Using the -fsigned-char flag makes the API look more like the one
generated on amd64 and arm64 and also more consistent with itself
(the syscall package represents chars using both int8 and uint8
types, the sys package will only ever use int8). Unfortunately it
also means that applications transitioning from the syscall package
to the unix package will see a different API on s390x which might
be confusing. I think the tradeoff is worth it though.

Change-Id: I40b90c18ed787e74ba7a2ebd004bd6bd1ba6279a
Reviewed-on: https://go-review.googlesource.com/23045Reviewed-by: Ian Lance Taylor <iant@golang.org>