runtime: gdb support: use parse_and_eval to get the goroutine id
runtime: gdb support: use parse_and_eval to get the goroutine id

This enables to loop over some goroutines, e.g. to print the
backtrace of goroutines 1 to 9:

        set $i = 1
        while $i < 10
        printf "backtrace of goroutine %d:\n", $i
        goroutine $i++ bt
        end

R=lvd, lvd
CC=golang-dev
http://codereview.appspot.com/6843071

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,webfd/go-zh,mhennings/marcohennings-go,d0f/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,webfd/go-zh,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,sanjosh/sanjos100-tipc,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,bryanxu/go-zh,webfd/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1