syscall: export Fsid.X__val on s390x

mkpost.go replaces all variables prefixed with 'X_' with '_' on s390x
because most of them do not need to be exposed. X__val is being used
by a third party library so it turns out we do need to expose it on
s390x (it is already exposed on all other Linux architectures).

Fixes #17298 and updates #18632.

Change-Id: Ic03463229a5f75ca41a4a4b50300da4b4d892d45
Reviewed-on: https://go-review.googlesource.com/30130Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/mkpost.go

@@ -18,6 +18,7 @@ import (
 	"log"
 	"os"
 	"regexp"
+	"strings"
 )
 
 func main() {
@@ -38,10 +39,16 @@ func main() {
 		re = regexp.MustCompile("Pad_cgo[A-Za-z0-9_]*")
 		s = re.ReplaceAllString(s, "_")
 
+		// We want to keep X__val in Fsid. Hide it and restore it later.
+		s = strings.Replace(s, "X__val", "MKPOSTFSIDVAL", 1)
+
 		// Replace other unwanted fields with blank identifiers.
 		re = regexp.MustCompile("X_[A-Za-z0-9_]*")
 		s = re.ReplaceAllString(s, "_")
 
+		// Restore X__val in Fsid.
+		s = strings.Replace(s, "MKPOSTFSIDVAL", "X__val", 1)
+
 		// Force the type of RawSockaddr.Data to [14]int8 to match
 		// the existing gccgo API.
 		re = regexp.MustCompile("(Data\\s+\\[14\\])uint8")

src/syscall/ztypes_linux_s390x.go

@@ -140,7 +140,7 @@ type Dirent struct {
 }
 
 type Fsid struct {
-	_ [2]int32
+	X__val [2]int32
 }
 
 type Flock_t struct {