* gold-threads.h (Once): Conditionalize member was_run_lock_ on
[ENABLE_THREADS && __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4], matching
its only use.
* gold-threads.cc (Once::Once): Likewise conditionalize initializer.
(class Initialize_lock): Rewrite as child of Once.
* gold-threads.cc (class Once_initialize): Define.
(once_pointer_control): New static variable.
(once_pointer, once_arg): New static variables.
(c_run_once): New static function.
(Once::Once, Once::run_once, Once::internal_run): New functions.
(class Initialize_lock_once): Remove.
(initialize_lock_control): Remove.
(initialize_lock_pointer): Remove.
(initialize_lock_once): Remove.
(Initialize_lock::Initialize_lock): Move to gold-threads.h.
(Initialize_lock::initialize): Rewrite.
(Initialize_lock::do_run_once): New function.
* archive.cc (Archive::interpret_header): Only clear name if it is
not already empty.
* fileread.cc: Include "gold-threads.h"
(file_counts_lock): New static variable.
(file_counts_initialize_lock): Likewise.
(File_read::release): Only increment counts when using --stats.
Use a lock around the increment.
* parameters.cc (class Set_parameters_target_once): Define.
(set_parameters_target_once): New static variable.
(Parameters::Parameters): Move here from parameters.h.
(Parameters::set_target): Rewrite.
(Parameters::set_target_once): New function.
(Parameters::clear_target): Move here and rewrite.
* parameters.h (class Parameters): Update declarations. Add
set_parameters_target_once_ field.
(Parameters::Parameters): Move to parameters.cc.
(Parameters::clear_target): Likewise.
* readsyms.cc (Read_symbols::do_group): Create a Start_group
task.
(Start_group::~Start_group): New function.
(Start_group::is_runnable): New function.
(Start_group::locks, Start_group::run): New functions.
(Finish_group::run): Change saw_undefined to size_t.
* readsyms.h (class Start_group): Define.
(class Finish_group): Change saw_undefined_ field to size_t.
(Finish_group::Finish_group): Remove saw_undefined and
this_blocker parameters. Change all callers.
(Finish_group::set_saw_undefined): New function.
(Finish_group::set_blocker): New function.
* symtab.h (class Symbol_table): Change saw_undefined to return
size_t. Change saw_undefined_ field to size_t.
* target-select.cc (Set_target_once::do_run_once): New function.
(Target_selector::Target_selector): Initialize set_target_once_
field. Don't initialize lock_ and initialize_lock_ fields.
(Target_selector::instantiate_target): Rewrite.
(Target_selector::set_target): New function.
* target-select.h (class Set_target_once): Define.
(class Target_selector): Update declarations. Make
Set_target_once a friend. Remove lock_ and initialize_lock_
fields. Add set_target_once_ field.
