build: change all.bash output to be more markdown-friendly

People keep pasting all.bash output into GitHub bugs, which turns
the # lines into <h1> headlines. Add some more #s so that the
bug reports are more readable. Not ideal but seems like the best
of a few bad options.

Change-Id: I4c69930ec304b2d504d7cd66221281a8577b87ae
Reviewed-on: https://go-review.googlesource.com/1286
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>