runtime: convert traceback*.c to Go

The two converted files were nearly identical.
Instead of continuing that duplication, I merged them
into a single traceback.go.

Tested on arm, amd64, amd64p32, and 386.

LGTM=r
R=golang-codereviews, remyoudompheng, dave, r
CC=dvyukov, golang-codereviews, iant, khr
https://codereview.appspot.com/134200044