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

Update #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 #7052
Update #6963
Fixes #6698

LGTM=rsc
R=golang-codereviews, josharian, rsc
CC=golang-codereviews
https://golang.org/cl/53590043