4 KB is a bit too small in some situations (e.g. panic during a
template execution), and ends up with an unhelpfully-truncated trace.
64 KB should be much more likely to capture the useful information.
There's not a garbage generation issue, since this code should only
be triggered when there's something seriously wrong with the program.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/63520043