log: expose Writer() method of the standard logger

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>