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, 44 insertions, 0 deletions

diff --git a/binutils/testsuite/binutils-all/compress.exp b/binutils/testsuite/binutils-all/compress.exp
index c88da74..4e74c82 100644
--- a/binutils/testsuite/binutils-all/compress.exp
+++ b/binutils/testsuite/binutils-all/compress.exp
@@ -576,6 +576,50 @@ if { [regexp_diff objdump.out $srcdir/$subdir/dw2-3gabi.W] } then {
     pass "$testname"
 }
 
+if { [binutils_assemble_flags $srcdir/$subdir/dw2-1.S ${compressedfile}zstd.o --compress-debug-sections=zstd] } then {
+    set testname "objcopy compress debug sections with zstd"
+    set got [binutils_run $OBJCOPY "--compress-debug-sections=zstd ${testfile}.o ${copyfile}zstd.o"]
+    if ![string match "" $got] then {
+	fail "objcopy ($testname)"
+	return
+    }
+    send_log "cmp ${compressedfile}zstd.o ${copyfile}zstd.o\n"
+    verbose "cmp ${compressedfile}zstd.o ${copyfile}zstd.o"
+    set src1 ${compressedfile}zstd.o
+    set src2 ${copyfile}zstd.o
+    set status [remote_exec build cmp "${src1} ${src2}"]
+    set exec_output [lindex $status 1]
+    set exec_output [prune_warnings $exec_output]
+    if ![string match "" $exec_output] then {
+	send_log "$exec_output\n"
+	verbose "$exec_output" 1
+	fail "objcopy ($testname)"
+    } else {
+	pass "objcopy ($testname)"
+    }
+
+    set testname "objcopy decompress compressed debug sections with zstd"
+    set got [binutils_run $OBJCOPY "--decompress-debug-sections ${compressedfile}zstd.o ${copyfile}zstd.o"]
+    if ![string match "" $got] then {
+	fail "objcopy ($testname)"
+	return
+    }
+    send_log "cmp ${testfile}.o ${copyfile}zstd.o\n"
+    verbose "cmp ${testfile}.o ${copyfile}zstd.o"
+    set src1 ${testfile}.o
+    set src2 ${copyfile}zstd.o
+    set status [remote_exec build cmp "${src1} ${src2}"]
+    set exec_output [lindex $status 1]
+    set exec_output [prune_warnings $exec_output]
+    if ![string match "" $exec_output] then {
+	send_log "$exec_output\n"
+	verbose "$exec_output" 1
+	fail "objcopy ($testname)"
+    } else {
+	pass "objcopy ($testname)"
+    }
+}
+
 proc convert_test { testname  as_flags  objcop_flags } {
     global srcdir
     global subdir