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

[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