cmd/dist: gcc is not a valid compiler on plan 9

cmd/dist/plan9.c should probably put 6c or 8c in defaultcc depending on if this is amd64 or 386 respectively.

```
; ./cmd/dist/dist env -9
CC='gcc'
CC_FOR_TARGET='gcc'
GOROOT='/tmp/plan9-amd64'
GOBIN='/tmp/plan9-amd64/bin'
GOARCH='amd64'
GOOS='plan9'
GOHOSTARCH='amd64'
GOHOSTOS='plan9'
GOTOOLDIR='/tmp/plan9-amd64/pkg/tool/plan9_amd64'
GOCHAR='6'
```
