os.Getwd: wrap error on windows and plan9

The errors from os are supposed to be wrapped to add some context,
but in this particular case a raw syscall error is returned.

Change-Id: I1b98dbd7b385c5c1cea79a1b0ec7201ca9bdca40
Reviewed-on: https://go-review.googlesource.com/c/go/+/606657


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/os/getwd.go

@@ -21,7 +21,8 @@ var getwdCache struct {
 // Getwd may return any one of them.
 func Getwd() (dir string, err error) {
 	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
-		return syscall.Getwd()
+		dir, err = syscall.Getwd()
+		return dir, NewSyscallError("getwd", err)
 	}
 
 	// Clumsy but widespread kludge: