The Go 1.12 introduced Writer() method for logger objects, but
it was not exposed as log package function for standard logger.
This commit adds such Writer() function.

Change-Id: Ia81b1524839fe05c152ecb5eaef047a076349fea
GitHub-Last-Rev: dc152ea641dd928178dbd921e2d0f6361661a0d6
GitHub-Pull-Request: golang/go#31009
Reviewed-on: https://go-review.googlesource.com/c/go/+/168920
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>