libgo: Use DejaGNU when testing a cross-compiler.

http://gcc.gnu.org/PR56017

R=iant
CC=gofrontend-dev
https://codereview.appspot.com/7308050
runtime: Fix argument passed to forcegchelper.

http://gcc.gnu.org/PR56172

R=iant
CC=gofrontend-dev
https://codereview.appspot.com/7303050