Dwarf: improve x86_64 backend debug info

[jacobly0]

$ ./zig run repro.zig
panic: crash
Unwind error at address `exe:0x80364c0` (error.MissingFDE), trace may be incomplete

repro.zig:2:5: 0x8000cf3 in main (repro.zig)
    @panic("crash");
    ^
lib/std/start.zig:575:22: 0x8000697 in posixCallMainAndExit (repro.zig)
            root.main();
                     ^
lib/std/start.zig:253:5: 0x800001a in _start (repro.zig)
    asm volatile (switch (native_arch) {
    ^
Aborted (core dumped)

Closes #17811