unix: allow overriding GOOS using GOOS_TARGET in mkpost.go

mkpost.go is platform independent and can in principle be run on a GOOS
other than the one we're generating the syscall wrappers for. Allow
overriding GOOS by setting GOOS_TARGET, similar to other generator
programs in the repo. This e.g. allows testing mkpost.go changes on a
different GOOS.

Follows CL 256278 which did the same for mksyscall.go

Change-Id: Ib99aa5cd266f7d27543cf9433cfb028f367eef63
Reviewed-on: https://go-review.googlesource.com/c/sys/+/484636
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>

Author: tklauser

unix/mkpost.go

@@ -24,7 +24,10 @@ import (
 
 func main() {
 	// Get the OS and architecture (using GOARCH_TARGET if it exists)
-	goos := os.Getenv("GOOS")
+	goos := os.Getenv("GOOS_TARGET")
+	if goos == "" {
+		goos = os.Getenv("GOOS")
+	}
 	goarch := os.Getenv("GOARCH_TARGET")
 	if goarch == "" {
 		goarch = os.Getenv("GOARCH")