cmd/go: fix build -n output when using swig
cmd/go: fix build -n output when using swig

$INTBITS will not be defined, of course, but that's the best we can do.

Fixes issue 5978.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13253048

apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme