go.bbclass: clean up CGO_xxx settings

go.bbclass: clean up CGO_xxx settings

* use conditional assignment for  the CGO_xxx
  variables, so they can be overridden more easily
* remove the TOOLCHAIN_OPTIONS and TARGET_CC_ARCH
  references, since those are already present in
  CC and CXX
* remove the TARGET_ prefix so the values are
  appropriate for native, nativesdk, etc. builds
* move the GOROOT export away from the CGO settings
  and closer to its definition

Signed-off-by: Matt Madison <matt@madison.systems>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>