weak when resolving to a dynamic def.
(Symbol_table::should_override): Add adjust_dyndef flag; set it
for weak undef/dynamic def cases. Adjust callers.
* symtab.cc (Symbol::init_fields): Initialize undef_binding_set_ and
undef_binding_weak_.
(Symbol_table::sized_write_globals): Adjust symbol binding.
(Symbol_table::sized_write_symbol): Add binding parameter.
* symtab.h (Symbol::set_undef_binding): New method.
(Symbol::is_undef_binding_weak): New method.
(Symbol::undef_binding_set_, Symbol::undef_binding_weak_): New members.
(Symbol_table::should_override): Add new parameter.
(Symbol_table::sized_write_symbol): Add new parameter.
* testsuite/weak_undef_file1.cc: Add new test case.
* testsuite/weak_undef_file2.cc: Fix header comment.
* testsuite/weak_undef_test.cc: Add new test case.
