os: use the syscall version of Getwd for Plan 9

In Plan 9, each OS thread has its own independent working directory,
so the Go runtime for Plan 9 needs to coordinate Chdir and Getwd
operations to keep the working directory consistent for all goroutines.

The function os.Getwd in Plan 9 should always call syscall.Getwd
to retrieve the common working directory.  Failure to do this was
the cause of (at least some of) the intermittent failures in the
Plan 9 builders with a seemingly spurious "file does not exist"
message, when a thread's working directory had been removed in
another thread.

Change-Id: Ifb834ad025ee39578234ad3b04d08bc98e939291
Reviewed-on: https://go-review.googlesource.com/89575Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/getwd.go

@@ -24,7 +24,7 @@ var useSyscallwd = func(error) bool { return true }
 // reached via multiple paths (due to symbolic links),
 // Getwd may return any one of them.
 func Getwd() (dir string, err error) {
-	if runtime.GOOS == "windows" {
+	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
 		return syscall.Getwd()
 	}