go/build: add AIX to "go build" command known OS

Issue golang/go#19200

Change-Id: Iceedd9630b0febd7af4c53d2ac29bb019eda60a8
Reviewed-on: https://go-review.googlesource.com/38511
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/libgo/go/go/build/syslist.go b/libgo/go/go/build/syslist.go
index ea316ea..fe9820e 100644
--- a/libgo/go/go/build/syslist.go
+++ b/libgo/go/go/build/syslist.go
@@ -4,5 +4,5 @@
 
 package build
 
-const goosList = "android darwin dragonfly freebsd linux nacl netbsd openbsd plan9 solaris windows zos "
+const goosList = "aix android darwin dragonfly freebsd linux nacl netbsd openbsd plan9 solaris windows zos "
 const goarchList = "386 amd64 amd64p32 arm armbe arm64 arm64be alpha m68k ppc64 ppc64le mips mipsle mips64 mips64le mips64p32 mips64p32le mipso32 mipsn32 mipsn64 mipso64 ppc s390 s390x sparc sparc64 "