[gdb/symtab] Fix gdb.base/fission-macro.exp with unix/-m32

When running test-case gdb.base/fission-macro.exp on openSUSE Tumbleweed
(using gcc 14) with target board unix/-m32, I get:
...
(gdb) info macro FIRST^M
Defined at /data/vries/gdb/src/gdb/testsuite/gdb.base/fission-macro.c:0^M
-DFIRST=1^M
(gdb) FAIL: $exp: \
  dwarf_version=5: dwarf_bits=32: strict_dwarf=0: info macro FIRST
...
instead of the expected:
...
(gdb) info macro FIRST^M
Defined at /data/vries/gdb/src/gdb/testsuite/gdb.base/fission-macro.c:18^M
(gdb) PASS: $exp: \
  dwarf_version=5: dwarf_bits=32: strict_dwarf=0: info macro FIRST
...

A dwarf-5 .debug_str_offsets section starts with a header consisting of:
- an initial length (4 bytes for 32-bit and 12 bytes for 64-bit),
- a 2 byte version string, and
- 2 bytes padding
so in total 8 bytes for 32-bit and 16 bytes for 64-bit.

This offset is calculated here in dwarf_decode_macros:
...
      str_offsets_base = cu->header.addr_size;
...
which is wrong for both dwarf-5 cases (and also happens to be wrong for
dwarf-4).

Fix this by computing str_offsets_base correctly for dwarf-5, for both the
32-bit and 64-bit case.

Likewise, fix this for dwarf-4, using str_offsets_base 0.  We can only test
this with gcc-15, because gcc 14 and earlier don't have the fix for
PR debug/115066.

Tested on x86_64-linux.

Tested test-case using a current gcc trunk build, and gcc 14.

Approved-By: Tom Tromey <tom@tromey.com>

PR symtab/31897
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31897

gdb/dwarf2/read.c

@@ -20832,7 +20832,17 @@ dwarf_decode_macros (struct dwarf2_cu *cu, unsigned int offset,
       str_offsets_section = &cu->dwo_unit->dwo_file
 			       ->sections.str_offsets;
       str_section = &cu->dwo_unit->dwo_file->sections.str;
-      str_offsets_base = cu->header.addr_size;
+      if (cu->header.version <= 4)
+	str_offsets_base = 0;
+      else
+	{
+	  bfd *abfd = str_offsets_section->get_bfd_owner ();
+	  unsigned int bytes_read = 0;
+	  read_initial_length (abfd, str_offsets_section->buffer, &bytes_read,
+			       false);
+	  const bool is_dwarf64 = bytes_read != 4;
+	  str_offsets_base = is_dwarf64 ? 16 : 8;
+	}
     }
   else
     {

gdb/testsuite/gdb.base/fission-macro.exp

@@ -66,8 +66,12 @@ proc do_tests { dwarf_version dwarf_bits strict_dwarf } {
     gdb_test "info macro FOURTH" "#define FOURTH 4"
 }
 
-foreach_with_prefix dwarf_version {5} {
-    foreach_with_prefix dwarf_bits {32} {
+foreach_with_prefix dwarf_version {4 5} {
+    if { $dwarf_version == 4 && [gcc_major_version] < 15 } {
+	# Gcc does not contain fix for PR debug/115066.
+	continue
+    }
+    foreach_with_prefix dwarf_bits {32 64} {
 	foreach_with_prefix strict_dwarf {0 1} {
 	    do_tests $dwarf_version $dwarf_bits $strict_dwarf
 	}