cmd/vet: use vet-specific export data to record detected printf wrappers

This CL takes advantage of the ability to record vet-specific export data,
added in CL 108558, to save information about observed printf wrappers.
Then calls to those wrappers from other packages can be format-checked.
This found a few real mistakes using previously-unrecognized printf
wrappers in cmd/compile. It will no doubt find real mistakes in external code.

Change-Id: I9c29c92d89bbdc984571a174a96e6054585e9cd4
Reviewed-on: https://go-review.googlesource.com/108559
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: rsc