Free sframe encoder context

Like the patch to free sframe decoder data, this also needs to ensure
the function doing the free is passed the actual context address.

bfd/
	* elf-sframe.c (_bfd_elf_write_section_sframe): Don't pass
	address of local var to sframe_encoder_free, pass the actual
	context address.
	* elf64-s390.c (elf_s390_link_hash_table_free): New function.
	(elf_s390_link_hash_table_create): Set hash_table_free.  Tidy
	zmalloc call.
	(_bfd_s390_elf_write_sframe_plt): Don't pass address of local
	var to sframe_encoder_free, pass the actual context address.
	* elflink.c: Include sframe-api.h.
	(_bfd_elf_link_hash_table_free): Free sframe encoder data.
	* elfxx-x86.c (elf_x86_link_hash_table_free): Likewise.
	(_bfd_x86_elf_write_sframe_plt): Don't pass address of local
	var to sframe_encoder_free, pass the actual context address.
libsframe/
	* sframe.c (sframe_encode): Free context on error return path.