runtime: explicit init of runtime-gdb helpers

If using other gdb python scripts loaded before Go's gdb-runtime.py
and that have a different init prototype:
Traceback (most recent call last):
  File "/usr/lib/go/src/pkg/runtime/runtime-gdb.py", line 446, in <module>
    k()
TypeError: __init__() takes exactly 3 arguments (1 given)

The problem is that gdb keeps all python scripts in the same namespace,
so vars() contains them. To avoid that, load helpers one by one.

R=iant, rsc
CC=gobot, golang-dev
https://golang.org/cl/9752044