unix: create types_illumos.go

create a new types_illumos.go for illumos specific constants
and structs, bring in those needed for wireguard-go

Change-Id: I68117abcae66127b617faea1f4cc2568b8b28d58
GitHub-Last-Rev: 3003e04e7e1c8d0e0677a98b89dac3f682a1ed22
GitHub-Pull-Request: golang/sys#100
Reviewed-on: https://go-review.googlesource.com/c/sys/+/301713
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/mkall.sh

@@ -199,7 +199,7 @@ illumos_amd64)
         mksyscall="go run mksyscall_solaris.go"
 	mkerrors=
 	mksysnum=
-	mktypes=
+	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
 	;;
 *)
 	echo 'unrecognized $GOOS_$GOARCH: ' "$GOOSARCH" 1>&2