go: new host package

Add a new package 'go' which builds the host cross compiler and
libraries for the go programming language.

Signed-off-by: Geoff Levand <geoff@infradead.org>
[Thomas:
 - Put the computation of GO_GOARM inside the ifeq ($(BR2_arm),y)
   condition rather than duplicating this condition.
 - Remove the GO_GOARCH=unknown case, since there is no way to fall in
   this case as only supported architectures can use host-go.
 - Remove the GO_GOARM=unknown case, since we are sure that only
   ARMv5/6/7 will use host-go.
 - Rename HOST_GO_FINAL to HOST_GO_ROOT, since it's really the "root"
   of the Go installation.
 - Remove visible Config.in.host option.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>