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

src/pkg/runtime/runtime-gdb.py

@@ -375,6 +375,7 @@ class GoroutineCmd(gdb.Command):
 
 	def invoke(self, arg, from_tty):
 		goid, cmd = arg.split(None, 1)
+		goid = gdb.parse_and_eval(goid)
 		pc, sp = find_goroutine(int(goid))
 		if not pc:
 			print "No such goroutine: ", goid