This patch adds zstd support to libbacktrace, to support the new
linker option --compress-debug-sections=zstd.

The zstd format is fairly complicated, so it's likely that there are
some bugs here.  It does pass the tests, at least.

Unfortunately this decompressor only runs at about 1/3 the speed to
the zstd library decompressor.  Still, it's smaller and simpler, and I
think it uses less memory.  Plus of course it uses the signal-safe
libbacktrace memory allocator.  Perhaps people can make a bit faster
over time.

Bootstrapped and ran libbacktrace and Go tests while using a linker
that compressed using zstd.

Committed to mainline.

Ian

Support decompressing --compress-debug-sections=zstd.
* configure.ac: Check for zstd library and
--compress-debug-sections=zstd linker option.
* Makefile.am (zstdtest_*): New targets.
(zstdtest_alloc_*, ctestzstd_*): New targets.
(BUILDTESTS): Add zstdtest, zstdtest_alloc, ctestzstd as
appropriate.
* elf.c (ELFCOMPRESS_ZSTD): Define.
(elf_fetch_bits): Rename from elf_zlib_fetch.  Update uses.
(elf_fetch_bits_backward): New static function.
(ZLIB_HUFFMAN_*): Rename from HUFFMAN_*.  Update uses.
(ZLIB_TABLE_*): Rename from ZDEBUG_TABLE_*.  Update uses.
(ZSTD_TABLE_*): Define.
(struct elf_zstd_fse_entry): Define.
(elf_zstd_read_fse): New static function.
(elf_zstd_build_fse): Likewise.
(lit): Define if BACKTRACE_GENERATE_ZSTD_FSE_TABLES.
(match, offset, next, print_table, main): Likewise.
(elf_zstd_lit_table): New static const array.
(elf_zstd_match_table, elf_zstd_offset_table): Likewise.
(elf_zstd_read_huff): New static function.
(struct elf_zstd_seq_decode): Define.
(elf_zstd_unpack_seq_decode): New static function.
(ZSTD_LIT_*): Define.
(struct elf_zstd_literals): Define.
(elf_zstd_literal_output): New static function.
(ZSTD_LITERAL_LENGTH_BASELINE_OFFSET): Define.
(elf_zstd_literal_length_baseline): New static const array.
(elf_zstd_literal_length_bits): Likewise.
(ZSTD_MATCH_LENGTH_BASELINE_OFFSET): Define.
(elf_zstd_match_length_baseline): New static const array.
(elf_zstd_match_length_bits): Likewise.
(elf_zstd_decompress): New static function.
(ZDEBUG_TABLE_SIZE): New definition.
(elf_uncompress_chdr): Support ELF_COMPRESS_ZSTD.
(backtrace_uncompress_zstd): New function.
(elf_add): Use ZLIB_TABLE_SIZE for zlib-gnu sections.
* internal.h (backtrace_uncompress_zstd): Declare.
* zstdtest.c: New file.
* configure, config.h.in, Makefile.in: Regenerate.