Fix up commit 31d1226, "Avoid aborting inside addr2line.c".
Source code informations did not appear in C level backtrace since
that change.

C99 does not allow this syntax.

According to @naruse, this `kprintf` was to avoid some segfaults, but
turned out it seemed another issue after all.

addr2line.c: fix DW_FORM_ref_addr parsing for DWARF 2

This fixes a crash when retrieving backtrace info with YJIT enabled on
macOS with Rust 1.71.0. Since Rust 1.71.0, the DWARF info generated by
the Rust compiler uses DW_FORM_ref_addr instead of DW_FORM_ref4 for
pointers to other DIEs.
DW_FORM_ref_addr representation in DWARF 2 is different from DWARF 3+,
so we need to handle it separately.
This patch fixes the parsing of DW_FORM_ref_addr for DWARF 2, which is
the default DWARF version Rustc uses on macOS.

See the DWARF 2.0.0 spec, section 7.5.4 Attribute Encodings
https://dwarfstd.org/doc/dwarf-2.0.0.pdf

https://bugs.ruby-lang.org/issues/19789

Clang 17 does not know warning group '-Wgnu-empty-initializer'.

DW_FORM_GNU_ref_alt and DW_FORM_GNU_strp_alt refer to data stored in an
external ELF file specified by a .gnu_debugaltlink attribute. These
attributes are generated by dwz(1), which extracts DWARF data common
amongst several files and stores it in a single, new file. It leaves
behind these two forms in the original file to point at the new, common
data.

We don't support actually reading the .gnu_debugaltlink file in
addr2line.c (and maybe we don't really need to), but we do need to know
how to read the actual value of these forms so we can skip over the
right number of bytes and not lose track of where we are in the CU.