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
https://golang.org/cl/6843071