Fix GC heap corruption on ARM. (#1389)

Fix GC heap corruption on ARM. (#1389)

The allocate_in_free code path in allocate_in_expanded_heap incorrectly calculated the large (double) alignment padding size when limiting the plug size (SHORT_PLUGS) if set_padding_on_saved_p was true:

    set_padding_in_expand (old_loc, set_padding_on_saved_p, pinned_plug_entry); // Sets the padding flag on the saved plug
    ...
    pad += switch_alignment_size (is_plug_padded (old_loc)); // Reads the padding flag from the old (different!) plug

That caused access violation during a later heap walk since the g_gc_pFreeObjectMethodTable pointer marking the gap was not placed at the right address.

Change-Id: I10a5fa443b3f5a614909983cd3c06d5420d22858