lookupSymbol (PEi386-only): When chasing down a label in a DLL, try
removing the initial underscore (if any). I've prefixed the comment
next to the change with the label 'HACK', as I'm not sure whether
this is a name mangling operation you should always do when going
from the import library name to the DLL-bound thing.
(Julian, could you have a look?)

Fixes probs with 'foreign import'ing DLL-bound symbols.