* symtab.h (Symbol_table::gc_mark_symbol_for_shlib): Rename to
gc_mark_symbol.
* symtab.cc (Symbol_table::gc_mark_symbol_for_shlib): Rename to
gc_mark_symbol.
Change to just keep the section associated with symbol.
(Symbol_table::add_from_relobj): Mark symbols as not garbage when
they are externally visible and --export-dynamic is turned on.
(Symbol_table::gc_mark_dyn_syms): Call gc_mark_symbol.