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/89575
Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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