elf: Fix a memory leak in _bfd_elf_add_dynamic_entry

elf: Fix a memory leak in _bfd_elf_add_dynamic_entry

Normally, the section contents is allocated by bfd_alloc which is freed
when the object is closed.  But the .dynamic section contents is allocated
by bfd_realloc, which should be freed by calling free.  Add a dynamic
field to elf_link_hash_table for the .dynamic section and free its
contents in _bfd_elf_link_hash_table_free.

	* elf-bfd.h (elf_link_hash_table): Add dynamic.
	* elflink.c (_bfd_elf_link_create_dynamic_sections): Set the
	dynamic field in elf_link_hash_table.
	(_bfd_elf_add_dynamic_entry): Use hash_table->dynamic.
	(_bfd_elf_strip_zero_sized_dynamic_sections): Likewise.
	(bfd_elf_add_dt_needed_tag): Likewise.
	(elf_finalize_dynstr): Likewise.
	(_bfd_elf_link_hash_table_free): Free htab->dynamic->contents.
	(bfd_elf_final_link): Use htab->dynamic.
	* elfxx-x86.c (_bfd_x86_elf_finish_dynamic_sections): Use
	htab->elf.dynamic.