[dev.cc] liblink: arrange for Prog* argument in vaddr

[dev.cc] liblink: arrange for Prog* argument in vaddr

The argument is unused in the C code but will be used in the Go translation,
because the Prog holds information needed to invoke the right meaning
of %A in the ctxt->diag calls in vaddr.

Change-Id: I501830f8ea0e909aafd8ec9ef5d7338e109d9548
Reviewed-on: https://go-review.googlesource.com/3041
Reviewed-by: Ian Lance Taylor <iant@golang.org>