libbacktrace: unpack literals into output buffer

	* elf.c (elf_fetch_backward_init): New static function.
	(ZSTD_TABLE_SIZE): Use huffman scratch space size rather than
	literal size.
	(ZSTD_TABLE_WORK_LIT_SIZE): Don't define.
	(elf_zstd_read_huff): Use elf_fetch_backward_init.
	(elf_zstd_read_literals): New static function.
	(ZSTD_LIT_RAW, ZSTD_LIT_RLE, ZSTD_LIT_HUFF): Don't define.
	(struct elf_zstd_literals): Don't define.
	(elf_zstd_literal_output): Remove static function.
	(elf_zstd_decompress): Use elf_fetch_backward_init and
	elf_zstd_read_literals.  Rewrite literal copying.<