x/sys/unix: go1.11 on mips64le fails at TestFstatat on Debian build [1.11 backport]

[gopherbot]

@anthonyfok requested issue #29401 to be considered for backport to the next 1.11 minor release.

@gopherbot please consider this for backport to 1.11 so that ./all.bash can complete TestFstatat successfully on linux/mips64{,le}. Thanks!

[gopherbot]

Change https://golang.org/cl/155751 mentions this issue: [release-branch.go1.11] unix: fix Fstatat by using fillStat_t on linux/mips64x

[ianlancetaylor]

There's no need for a 1.11 release branch in the golang.org/x/sys repo.