When set to false, the -dolinkobj flag instructs the compiler
not to generate or emit linker information.

This is handy when you need the compiler's export data,
e.g. for use with go/importer,
but you want to avoid the cost of full compilation.

This must be used with care, since the resulting
files are unusable for linking.

This CL interacts with #18369,
where adding gcflags and ldflags to buildid has been mooted.
On the one hand, adding gcflags would make safe use of this
flag easier, since if the full object files were needed,
a simple 'go install' would fix it.
On the other hand, this would mean that
'go install -gcflags=-dolinkobj=false' would rebuild the object files,
although any existing object files would probably suffice.

Change-Id: I8dc75ab5a40095c785c1a4d2260aeb63c4d10f73
Reviewed-on: https://go-review.googlesource.com/37384
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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>