runtime: reenable bad pointer check in GC

[rsc]

runtime/mbitmap.go says:

    // The following ensures that we are rigorous about what data
    // structures hold valid pointers.
    // TODO(rsc): Check if this still happens.
    if false {
        // Still happens sometimes. We don't know why.
        printlock()
        print("runtime:objectstart Span weird: p=", hex(p), " k=", hex(k))
        if s == nil {
            print(" s=nil\n")
        } else {
            print(" s.start=", hex(s.start<<_PageShift), " s.limit=", hex(s.limit), " s.state=", s.state, "\n")
        }
        printunlock()
        throw("objectstart: bad pointer in unexpected span")
    }

This needs to happen for Go 1.5.
We broke a lot of programs in Go 1.4 by insisting on this
(for good reasons), and we don't want relapses during Go 1.5.

[aclements]

Duplicate of #9880.