package/go: fix cross-compilation settings

This patch fixes a bug with the BR2_TOOLCHAIN_HAS_THREADS variable
handling which causes CGO_ENABLED to be always 0.

Furthermore, it fixes the cross compilation options for the go
compiler: setting CGO_ENABLED should be done only for the target
compiler not the host one.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Christian Stewart <christian@paral.in>