go: remove powerpc64 big-endian from supported architectures

cgo currently doesn't properly support powerpc64 big-endian, as noted
in https://github.com/golang/go/issues/13192, and indeed, we have a
large number of build failures of Go packages on this
architecture. This commit therefore disables Go on PowerPC64
big-endian (PowerPC64 little-endian is fine).

Fixes:

  http://autobuild.buildroot.net/results/a6e9bac0a735f48d0ba0af081aeac4ed9fdfaca7/
  (flannel)

  http://autobuild.buildroot.net/results/230f52bc35f437836c7a76d4b58ef454635ee0d3/
  (docker-containerd)

  http://autobuild.buildroot.net/results/77c31d6e8f5efe3e024e27a160cf5d1d1952719e/
  (runc)

  http://autobuild.buildroot.net/results/a87b07417ea8bd81ffe27e5661b4359ddc0149ab/


  (docker-engine)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>