runtime: initialize traceback variables earlier

Our traceback code needs to know the PC of several special
functions, including goexit, mcall, etc.  Make sure that
these PCs are initialized before any traceback occurs.

Fixes issue 8766

LGTM=rsc
R=golang-codereviews, rsc, khr, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/145570043