syscall, internal/poll: fix build on js/wasm

Fixes #26014

Change-Id: I9d92414a9181c5d189e3e266666950656bf00406
Reviewed-on: https://go-review.googlesource.com/120576
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/poll/fd_unix.go

@@ -455,9 +455,10 @@ func DupCloseOnExec(fd int) (int, string, error) {
 		switch e1 {
 		case 0:
 			return int(r0), "", nil
-		case syscall.EINVAL:
-			// Old kernel. Fall back to the portable way
-			// from now on.
+		case syscall.EINVAL, syscall.ENOSYS:
+			// Old kernel, or js/wasm (which returns
+			// ENOSYS). Fall back to the portable way from
+			// now on.
 			atomic.StoreInt32(&tryDupCloexec, 0)
 		default:
 			return -1, "fcntl", e1

src/syscall/net_js.go

@@ -38,6 +38,13 @@ const (
 	SO_ERROR
 )
 
+// Misc constants expected by package net but not supported.
+const (
+	_ = iota
+	F_DUPFD_CLOEXEC
+	SYS_FCNTL = 500 // unsupported; same value as net_nacl.go
+)
+
 type Sockaddr interface {
 }