Currently, the symbol updater uses a pointer pointing to the
loader's payloads array. If the payloads slice grows (and moves),
the pointer may become stale and no longer point to the symbol's
actual payload. Specifically, consider
sb, sym := l.MakeSymbolUpdater(...)
// add a bunch of external symbols, which grows payload slice
sb.SetType(t)
l.SymType(sym) // may not return t
sb.SetType on line 3 may not have the desired effect, as
sb.extSymPayload may no longer point to the right payload. As a
result, the type we get on line 4 may be not the one we set.
Fix this by making the payload's address permanent. Once it is
allocated it will never move.