Extend pdp11-aout symbol table format and code for .stab symbols.

Extend pdp11-aout symbol table format and code for .stab symbols.

* bfd/pdp11.c (pdp11_external_nlist): Repurposed e_unused to e_desc.
(N_STAB, is_stab): Needed new function is_stab to disambiguate
normal vs. .stab symbol table type values, replacing N_STAB mask.
(translate_from_native_sym_flags): Determine correct section for
different .stab types.
(translate_to_native_sym_flags): Leave .stab types intact.
(translate_symbol_table): Error if symbol indicates overlay;
store desc field from .stab symbols.
(write_syms): Output desc field with symbol.
(aout_link_check_ar_symbols): Skip .stab symbols.
(aout_link_add_symbols): Correctly distinguish .stab symbols.
(aout_link_write_other_symbol): Write 0 for desk and ovly fields.
(aout_link_write_symbols): Write 0 for desk and ovly fields;
correctly distinguish .stab symbols and select calculate their
section and value; and copy desc and ovly fields from input symbol
to output symbol.