PR 10860
* options.h (class General_options): Add --warn-common.
* resolve.cc (Symbol_table::resolve): Handle --warn-common when
merging two common symbols.
(Symbol_table::should_override): Handle --warn-common when merging
a common symbol with a defined symbol. Use report_resolve_problem
for multiple definitions.
(Symbol_table::report_resolve_problem): New function.
* symtab.h (class Symbol_table): Declare report_resolve_problem.