Fix race condition causing assert in Eh_frame_hdr::do_sized_write().

Fix race condition causing assert in Eh_frame_hdr::do_sized_write().

2014-09-23  Taiju Tsuiki  <tzik@google.com>
            Cary Coutant  <ccoutant@google.com>

gold/
	* gold.cc (queue_final_tasks): Add Write_sections_task as a blocker
	on input_sections_blocker.
	* layout.cc (Write_sections_task::locks): Unblock
	input_sections_blocker_.
	* layout.h (Write_sections_task::Write_sections_task): Add
	input_sections_blocker.
	* testsuite/Makefile.am (exception_x86_64_bnd_test): Add gcctestdir/ld
	to DEPENDENCIES.
	* testsuite/Makefile.in: Regenerate.