Move the single file from internal/syscall to internal/syscall/unix,
to match the golang.org/x/sys layout.

Change-Id: I2fb2832b4cb22efc7666bd276f5401ac3e73dc40
Reviewed-on: https://go-review.googlesource.com/8972Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>