[gdb/testsuite] Fix gdb.dwarf2/dw2-lines.exp with -m32

[gdb/testsuite] Fix gdb.dwarf2/dw2-lines.exp with -m32

When running test-case gdb.dwarf2/dw2-lines.exp with target board -unix/-m32,
we run into another instance of PR28383, where the dwarf assembler generates
64-bit relocations which are not supported by the 32-bit assembler:
...
dw2-lines-dw.S: Assembler messages:^M
outputs/gdb.dwarf2/dw2-lines/dw2-lines-dw.S:76: Error: \
  cannot represent relocation type BFD_RELOC_64^M
...

Fix this by using _op_offset in _line_finalize_header.

Tested on x86_64-linux.