binutils, gdb: support zstd compressed debug sections

PR29397 PR29563: Add new configure option --with-zstd which defaults to
auto.  If pkgconfig/libzstd.pc is found, define HAVE_ZSTD and support
zstd compressed debug sections for most tools.

* bfd: for addr2line, objdump --dwarf, gdb, etc
* gas: support --compress-debug-sections=zstd
* ld: support ELFCOMPRESS_ZSTD input and --compress-debug-sections=zstd
* objcopy: support ELFCOMPRESS_ZSTD input for
  --decompress-debug-sections and --compress-debug-sections=zstd
* gdb: support ELFCOMPRESS_ZSTD input.  The bfd change references zstd
  symbols, so gdb has to link against -lzstd in this patch.

If zstd is not supported, ELFCOMPRESS_ZSTD input triggers an error.  We
can avoid HAVE_ZSTD if binutils-gdb imports zstd/ like zlib/, but this
is too heavyweight, so don't do it for now.

```
% ld/ld-new a.o
ld/ld-new: a.o: section .debug_abbrev is compressed with zstd, but BFD is not built with zstd support
...

% ld/ld-new a.o --compress-debug-sections=zstd
ld/ld-new: --compress-debug-sections=zstd: ld is not built with zstd support

% binutils/objcopy --compress-debug-sections=zstd a.o b.o
binutils/objcopy: --compress-debug-sections=zstd: binutils is not built with zstd support

% binutils/objcopy b.o --decompress-debug-sections
binutils/objcopy: zstd.o: section .debug_abbrev is compressed with zstd, but BFD is not built with zstd support
...
```

1 files changed, 11 insertions, 2 deletions

diff --git a/gas/as.c b/gas/as.c
index 6268779..35ad6b3 100644
--- a/gas/as.c
+++ b/gas/as.c
@@ -252,14 +252,14 @@ Options:\n\
   --alternate             initially turn on alternate macro syntax\n"));
 #ifdef DEFAULT_FLAG_COMPRESS_DEBUG
   fprintf (stream, _("\
-  --compress-debug-sections[={none|zlib|zlib-gnu|zlib-gabi}]\n\
+  --compress-debug-sections[={none|zlib|zlib-gnu|zlib-gabi|zstd}]\n\
                           compress DWARF debug sections using zlib [default]\n"));
   fprintf (stream, _("\
   --nocompress-debug-sections\n\
                           don't compress DWARF debug sections\n"));
 #else
   fprintf (stream, _("\
-  --compress-debug-sections[={none|zlib|zlib-gnu|zlib-gabi}]\n\
+  --compress-debug-sections[={none|zlib|zlib-gnu|zlib-gabi|zstd}]\n\
                           compress DWARF debug sections using zlib\n"));
   fprintf (stream, _("\
   --nocompress-debug-sections\n\
@@ -736,6 +736,15 @@ This program has absolutely no warranty.\n"));
 		flag_compress_debug = COMPRESS_DEBUG_GNU_ZLIB;
 	      else if (strcasecmp (optarg, "zlib-gabi") == 0)
 		flag_compress_debug = COMPRESS_DEBUG_GABI_ZLIB;
+	      else if (strcasecmp (optarg, "zstd") == 0)
+		{
+#ifdef HAVE_ZSTD
+		  flag_compress_debug = COMPRESS_DEBUG_ZSTD;
+#else
+		  as_fatal (_ ("--compress-debug-sections=zstd: gas is not "
+			       "built with zstd support"));
+#endif
+		}
 	      else
 		as_fatal (_("Invalid --compress-debug-sections option: `%s'"),
 			  optarg);