cmd/vendor: build broken on linux/arm after x/sys/unix update

[ALTree]

CL 153838 re-vendored golang.org/x/sys/unix, and that broke the linux/arm builder:

# cmd/vendor/golang.org/x/sys/unix_test [cmd/vendor/golang.org/x/sys/unix.test]
go/src/cmd/vendor/golang.org/x/sys/unix/syscall_linux_test.go:471:8: undefined: unix.SyncFileRange
go/src/cmd/vendor/golang.org/x/sys/unix/syscall_linux_test.go:480:8: undefined: unix.SyncFileRange
FAIL	cmd/vendor/golang.org/x/sys/unix [build failed]
2018/12/13 04:17:37 Failed: exit status 2

example:
https://build.golang.org/log/4fb5c10b30bdaa9b04a1b048ae9043afcd8160df

cc @4a6f656c

[bcmills]

CC @bradfitz @tklauser

[bradfitz]

@tklauser already added this to x/sys/unix in https://go-review.googlesource.com/c/sys/+/153939

This now needs to be re-vendored.

[gopherbot]

Change https://golang.org/cl/154117 mentions this issue: cmd/vendor: update vendored golang.org/x/sys/unix

[tklauser]

Before revendoring x/sys/unix I'd like to submit https://golang.org/cl/153938 to unbreak aix in the same update.