Using rsc.io/c2go repo revision 60c9302.

- Export a few symbols needed by assemblers.
- Implement Getgoroot etc directly, and add Getgoversion.
- Removes dependency on Go 1.4 go/build.
- Change magic history name <no name> to <pop>

The <pop> change requires adjustment to the liblink serializer.

Change-Id: If5fb52ac9e91d50805263070b3fc5cc05d8b7632
Reviewed-on: https://go-review.googlesource.com/3141Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>