Printf("%x", "abc") was "0x610x620x63"; is now "0x616263", which
is surely better.
Printf("% #x", "abc") is still "0x61 0x62 0x63".

Fixes #8080.

LGTM=bradfitz, gri
R=golang-codereviews, bradfitz, gri
CC=golang-codereviews
https://golang.org/cl/106990043