When running on the host platform,
the standard library has almost certainly already been built.
However, all other platforms will probably need building.
Use the new -dolinkobj=false flag to cmd/compile
to only build the export data instead of doing a full compile.

Having partial object files could be confusing for people
doing subsequent cross-compiles, depending on what happens with #18369.
However, cmd/vet/all will mainly be run by builders
and core developers, who are probably fairly well-placed
to handle any such confusion.

This reduces the time on my machine for a cold run of
'go run main.go -all' by almost half:

benchmark           old ns/op        new ns/op        delta
BenchmarkVetAll     240670814551     130784517074     -45.66%

Change-Id: Ieb866ffb2cb714b361b0a6104077652f8eacd166
Reviewed-on: https://go-review.googlesource.com/37385
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>