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, 10 insertions, 0 deletions

diff --git a/configure b/configure
index d75f47a..f14e0ef 100755
--- a/configure
+++ b/configure
@@ -785,6 +785,7 @@ ac_user_opts='
 enable_option_checking
 with_build_libsubdir
 with_system_zlib
+with_zstd
 enable_as_accelerator_for
 enable_offload_targets
 enable_gold
@@ -1567,6 +1568,8 @@ Optional Packages:
   --without-PACKAGE       do not use PACKAGE (same as --with-PACKAGE=no)
   --with-build-libsubdir=DIR  Directory where to find libraries for build system
   --with-system-zlib      use installed libz
+  --with-zstd             Support zstd compressed debug sections
+                          (default=auto)
   --with-mpc=PATH         specify prefix directory for installed MPC package.
                           Equivalent to --with-mpc-include=PATH/include plus
                           --with-mpc-lib=PATH/lib
@@ -2925,6 +2928,13 @@ if test x$with_system_zlib = xyes ; then
   noconfigdirs="$noconfigdirs zlib"
 fi
 
+
+# Check whether --with-zstd was given.
+if test "${with_zstd+set}" = set; then :
+  withval=$with_zstd;
+fi
+
+
 # Don't compile the bundled readline/libreadline.a if --with-system-readline
 # is provided.
 if test x$with_system_readline = xyes ; then