cmd/go: update private copy of goosList

This copies the change to goosList in CL 138115 to the private copy in
cmd/go.

The change introducing the private copy was apparently not made with
Gerrit, but can be seen at
https://github.com/golang/vgo/commit/08359e782fb601567c57f56beb540841c2416d92.
That change says "This is adapted from code in go/build and the rest
of cmd/go. At some point, we should deduplicate them."

Doing another copy for now, rather than something more complex
involving cmd/dist, pending that deduplication.

Change-Id: I9b6e1f63a3a68c002b60a9a97aa367c5cc7801c9
Reviewed-on: https://go-review.googlesource.com/c/143759
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/imports/build.go

@@ -207,5 +207,5 @@ func init() {
 	}
 }
 
-const goosList = "android darwin dragonfly freebsd js linux nacl netbsd openbsd plan9 solaris windows zos "
+const goosList = "aix android darwin dragonfly freebsd js 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 riscv riscv64 s390 s390x sparc sparc64 wasm "