If C_HIDEXT and C_AIX_WEAKEXT symbols aren't handled as globals by
coff_classify_symbol then we run into "warning: .. local symbol `some
garbage name' has no section". These are of course both global
symbols, but C_HIDEXT is like a local in some respects and returning
COFF_SYMBOL_LOCAL for C_HIDEXT keeps nm output looking the same.
Fixes these fails on rs6000-aix5.1: