syscall: add Hurd support

Loosely based on a patch by Svante Signell.

Change-Id: I7e7efa1cc295c800d03ed5a5eaf1b2e94b5cf3bf
Reviewed-on: https://go-review.googlesource.com/c/161518
Reviewed-by: Ian Lance Taylor <iant@golang.org>
9 files changed