[dev.cc] liblink: invoke go tool objwriter during writeobj

[dev.cc] liblink: invoke go tool objwriter during writeobj

This doesn't actually use objwriter for any real work.
It's just to check that objwriter is available.
The real work will be moved once the bootstrapping
mechanisms are working.

Change-Id: I5f41c8910c4b11b9d80cb0b0847ff9cb582fc2be
Reviewed-on: https://go-review.googlesource.com/3045
Reviewed-by: Ian Lance Taylor <iant@golang.org>