libbacktrace: rewrite and simplify main zstd loop

	* elf.c (ZSTD_TABLE_*): Use elf_zstd_fse_baseline_entry.
	(ZSTD_ENCODE_BASELINE_BITS): Define.
	(ZSTD_DECODE_BASELINE, ZSTD_DECODE_BASEBITS): Define.
	(elf_zstd_literal_length_base): New static const array.
	(elf_zstd_match_length_base): Likewise.
	(struct elf_zstd_fse_baseline_entry): Define.
	(elf_zstd_make_literal_baseline_fse): New static function.
	(elf_zstd_make_offset_baseline_fse): Likewise.
	(elf_zstd_make_match_baseline_fse): Likewise.
	(print_table, main): Use elf_zstd_fse_baseline_entry.
	(elf_zstd_lit_table, elf_zstd_match_table): Likewise.
	(elf_zstd_offset_table): Likewise.
	(struct elf_zstd_seq_decode): Likewise.  Remove use_rle and rle
	fields.
	(elf_zstd_unpack_seq_decode): Use elf_zstd_fse_baseline_entry,
	taking a conversion function.  Convert RLE to FSE.
	(elf_zstd_literal_length_baseline): Remove.
	(elf_zstd_literal_length_bits): Remove.
	(elf_zstd_match_length_baseline): Remove.
	(elf_zstd_match_length_bits): Remove.
	(elf_zstd_decompress): Use elf_zstd_fse_baseline_entry.  Rewrite
	and simplify main loop.