* options.cc: Include <cerrno> and <fstream>.
(General_options::finalize): Parse -retain-symbols-file tag.
* options.h: New flag.
(General_options): New method should_retain_symbol, new
variable symbols_to_retain.
* symtab.cc (Symbol_table::sized_finalize_symbol): Test
should_retain_symbol map.
* testsuite/Makefile.am (retain_symbols_file_test): New test.
* testsuite/Makefile.in: Regenerate.
* testsuite/retain_symbols_file_test.sh: New file.