cmd/vet: improve error message for cross-package assembly

cmd/vet: improve error message for cross-package assembly

bytes.Compare has its go prototype in package bytes,
but its implementation in package runtime.
vet used to complain that the prototype was missing.
Now instead:

runtime/asm_amd64.s:1483: [amd64] cannot check cross-package assembly function: Compare is in package bytes

Updates #11041

Change-Id: Ied44fac10d0916d7a34e552c02d052e16fca0c8c
Reviewed-on: https://go-review.googlesource.com/27153
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>