[llvm-objdump] --syms: make flags closer to GNU objdump

This fixes several issues. The behavior changes are:

A SHN_COMMON symbol does not have the 'g' flag.
An undefined symbol does not have 'g' or 'l' flag.
A STB_GLOBAL SymbolRef::ST_Unknown symbol has the 'g' flag.
A STB_LOCAL SymbolRef::ST_Unknown symbol has the 'l' flag.

Reviewed By: rupprecht

Differential Revision: https://reviews.llvm.org/D75659