go/build: reserve GOOS=zos for IBM z/OS

Closes #17528.

Change-Id: I2ba55ad4e41077808f882ed67a0549f0a00e25d0
Reviewed-on: https://go-review.googlesource.com/31596
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/build/syslist.go

@@ -4,5 +4,5 @@
 
 package build
 
-const goosList = "android darwin dragonfly freebsd linux nacl netbsd openbsd plan9 solaris windows "
+const goosList = "android darwin dragonfly freebsd linux nacl netbsd openbsd plan9 solaris windows zos "
 const goarchList = "386 amd64 amd64p32 arm armbe arm64 arm64be ppc64 ppc64le mips mipsle mips64 mips64le mips64p32 mips64p32le ppc s390 s390x sparc sparc64 "