runtime: gdb support: gracefully handle not being able to find types
runtime: gdb support: gracefully handle not being able to find types

The Dwarf info has the full typenames, the go *struct runtime.commonType
has the short name.  A more permanent fix would link the two together
but this way the user gets useable stack traces for now.

R=rsc
CC=golang-dev
http://codereview.appspot.com/5097046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang