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, 5 insertions, 0 deletions

diff --git a/ld/ld.texi b/ld/ld.texi
index eabbec8..9daed2e 100644
--- a/ld/ld.texi
+++ b/ld/ld.texi
@@ -2863,10 +2863,12 @@ but for most Linux based systems it will be @code{both}.
 @kindex --compress-debug-sections=zlib
 @kindex --compress-debug-sections=zlib-gnu
 @kindex --compress-debug-sections=zlib-gabi
+@kindex --compress-debug-sections=zstd
 @item --compress-debug-sections=none
 @itemx --compress-debug-sections=zlib
 @itemx --compress-debug-sections=zlib-gnu
 @itemx --compress-debug-sections=zlib-gabi
+@itemx --compress-debug-sections=zstd
 On ELF platforms, these options control how DWARF debug sections are
 compressed using zlib.
 
@@ -2880,6 +2882,9 @@ sets the SHF_COMPRESSED flag in the sections' headers.
 The @option{--compress-debug-sections=zlib} option is an alias for
 @option{--compress-debug-sections=zlib-gabi}.
 
+@option{--compress-debug-sections=zstd} compresses DWARF debug sections using
+zstd.
+
 Note that this option overrides any compression in input debug
 sections, so if a binary is linked with @option{--compress-debug-sections=none}
 for example, then any compressed debug sections in input files will be