gdb: Add partial python3 + go1.2 support to runtime-gdb.py
gdb: Add partial python3 + go1.2 support to runtime-gdb.py

Update issue 6963 Fixes pretty printing maps and updates
functions for interacting with $len(). goroutine $n bt
remains not working. Tested on gdb using python 2 and 3.
Fixes issue 7052
Update issue 6963
Fixes issue 6698

LGTM=rsc
R=golang-codereviews, josharian, rsc
CC=golang-codereviews
https://codereview.appspot.com/53590043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old