runtime: Correct handling of runtime.Callers skip parameter.

R=iant
CC=gofrontend-dev
https://codereview.appspot.com/7229077
syscall: Only run creds_test on GNU/Linux.

http://gcc.gnu.org/PR56171

R=iant
CC=gofrontend-dev
https://codereview.appspot.com/7283043