unix: re-export Fsid and Sigset_t Val member on Linux

As spotted by Brad Fitzpatrick, CL 106838 unexported the value field of
Fsid and Sigset_t, but these types would be useless without any exported
fields. Re-export them via an exception in mkpost.go

Change-Id: I30d3c3b81c8c1b6377f8aa55cad816e3486a4395
Reviewed-on: https://go-review.googlesource.com/107415
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/mkpost.go

@@ -42,6 +42,10 @@ func main() {
 		log.Fatal(err)
 	}
 
+	// Intentionally export __val fields in Fsid and Sigset_t
+	valRegex := regexp.MustCompile(`type (Fsid|Sigset_t) struct {(\s+)X__val(\s+\S+\s+)}`)
+	b = valRegex.ReplaceAll(b, []byte("type $1 struct {${2}Val$3}"))
+
 	// If we have empty Ptrace structs, we should delete them. Only s390x emits
 	// nonempty Ptrace structs.
 	ptraceRexexp := regexp.MustCompile(`type Ptrace((Psw|Fpregs|Per) struct {\s*})`)