Bug#803099: golang-ginkgo: FTBFS on arm64

[Edmund Grimley Evans]

I must have been thinking of an entirely different programming
language. For some reason I was thinking that the undefined syscall
was a run-time rather than a compile-time error. Clearly you can't use
Dup2 at all on arm64. Does that mean you need two additional source
files, one with "// +build arm64", one with "// +build !arm64", if
there isn't already a wrapper defined somewhere else?