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
...
```

config/ChangeLog:

	* zstd.m4: New file.

ChangeLog:
	* configure: Regenerate.
	* configure.ac: Add --with-zstd.

config/zstd.m4

@@ -0,0 +1,23 @@
+dnl Copyright (C) 2022 Free Software Foundation, Inc.
+dnl This file is free software, distributed under the terms of the GNU
+dnl General Public License.  As a special exception to the GNU General
+dnl Public License, this file may be distributed as part of a program
+dnl that contains a configuration script generated by Autoconf, under
+dnl the same distribution terms as the rest of that program.
+
+dnl Enable features using the zstd library.
+AC_DEFUN([AC_ZSTD], [
+AC_ARG_WITH(zstd,
+  [AS_HELP_STRING([--with-zstd], [support zstd compressed debug sections (default=auto)])],
+  [], [with_zstd=auto])
+
+AS_IF([test "$with_zstd" != no],
+  [PKG_CHECK_MODULES(ZSTD, [libzstd], [
+    AC_DEFINE(HAVE_ZSTD, 1, [Define to 1 if zstd is enabled.])
+  ], [
+    if test "$with_zstd" = yes; then
+      AC_MSG_ERROR([--with-zstd was given, but pkgconfig/libzstd.pc is not found])
+    fi
+  ])
+  ])
+])

configure

@@ -800,6 +800,7 @@ ac_user_opts='
 enable_option_checking
 with_build_libsubdir
 with_system_zlib
+with_zstd
 enable_as_accelerator_for
 enable_offload_targets
 enable_offload_defaulted
@@ -1589,6 +1590,8 @@ 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
@@ -2962,6 +2965,13 @@ if test x$with_system_readline = xyes ; then
   noconfigdirs="$noconfigdirs readline"
 fi
 
+
+# Check whether --with-zstd was given.
+if test "${with_zstd+set}" = set; then :
+  withval=$with_zstd;
+fi
+
+
 # some tools are so dependent upon X11 that if we're not building with X,
 # it's not even worth trying to configure, much less build, that tool.
 

configure.ac

@@ -254,6 +254,9 @@ if test x$with_system_readline = xyes ; then
   noconfigdirs="$noconfigdirs readline"
 fi
 
+AC_ARG_WITH(zstd,
+[AS_HELP_STRING([--with-zstd], [Support zstd compressed debug sections (default=auto)])])
+
 # some tools are so dependent upon X11 that if we're not building with X, 
 # it's not even worth trying to configure, much less build, that tool.