runtime/pprof: adjust reported line numbers to show call sites
runtime/pprof: adjust reported line numbers to show call sites

This is the same logic used in the standard tracebacks.
The caller pc is the pc after the call, so except in the
fake "call" caused by a panic, back up the pc enough
that the lookup will use the previous instruction.

Fixes issue 4150.
Fixes issue 4151.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7317047

bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,scirelli/scirelli-go,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,d0f/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,scirelli/scirelli-go,webfd/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,mhennings/marcohennings-go