* gold.cc (queue_middle_gc_tasks): Use a separate blocker for each

	* gold.cc (queue_middle_gc_tasks): Use a separate blocker for each
	Read_relocs task.
	(queue_middle_tasks): Likewise, and also for Scan_relocs.  Run
	Allocate_commons_task first.
	* reloc.cc (Read_relocs::run): Pass next_blocker_ down to next
	task, rather than symtab_lock_.
	(Gc_process_relocs::~Gc_process_relocs): New function.
	(Gc_process_relocs::is_runnable): Check this_blocker_.
	(Gc_process_relocs::locks): Use next_blocker_ rather than
	blocker_.
	(Scan_relocs::~Scan_relocs): New function.
	(Scan_relocs::is_runnable): Check this_blocker_ rather than
	symtab_lock_.
	(Scan_relocs::locks): Drop symtab_lock_ and blocker_.  Add
	next_blocker_.
	* reloc.h (class Read_relocs): Drop symtab_lock_ and blocker_
	fields.  Add this_blocker_ and next_blocker_ fields.  Adjust
	constructor accordingly.
	(class Gc_process_relocs): Likewise.
	(class Scan_relocs): Likewise.
	* common.h (class Allocate_commons_task): Remove symtab_lock_
	field, and corresponding constructor parameter.
	* common.cc (Allocate_commons_tasK::is_runnable): Remove use of
	symtab_lock_.
	(Allocate_commons_task::locks): Likewise.