On Plan 9 gotraceback calls getenv calls malloc, and we gotraceback
on every call to gentraceback, which happens during garbage collection.
Honestly I don't even know how this works on Plan 9.
I suspect it does not, and that we are getting by because
no one has tried to run with $GOTRACEBACK set at all.

This will speed up all the other systems by epsilon, since they
won't call getenv and atoi repeatedly.

LGTM=bradfitz
R=golang-codereviews, bradfitz, 0intro
CC=golang-codereviews
https://golang.org/cl/85430046