bad dwarf debug info when using self-hosted x86 backend

[andrewrk]

zig version: 0.12.0-dev.1372+8457439a8

pub fn main() void {
    @panic("crash");
}

$ stage3/bin/zig run test.zig -fno-llvm -fno-lld
panic: crash
Unwind error at address `exe:0x8032a6e` (error.MissingFDE), trace may be incomplete

/home/andy/Downloads/zig/build-release/test.zig:2:0: 0x8000c70 in test.main (test.zig)
    @panic("crash");

/home/andy/Downloads/zig/lib/std/start.zig:369:0: 0x800062f in start.posixCallMainAndExit (test.zig)
    while (envp_optional[envp_count]) |_| : (envp_count += 1) {}

/home/andy/Downloads/zig/lib/std/start.zig:251:0: 0x800001a in start._start (test.zig)
    asm volatile (switch (native_arch) {

Aborted (core dumped)

Expected output:

$ stage3/bin/zig run test.zig -fllvm -flld
thread 411659 panic: crash
/home/andy/Downloads/zig/build-release/test.zig:2:5: 0x21dcd5 in main (test)
    @panic("crash");
    ^
/home/andy/Downloads/zig/lib/std/start.zig:573:22: 0x21d57c in posixCallMainAndExit (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:251:5: 0x21d0d1 in _start (test)
    asm volatile (switch (native_arch) {
    ^
???:?:?: 0x0 in ??? (???)
Aborted (core dumped)

Observations:

• Unwind error at address exe:0x8032a6e (error.MissingFDE), trace may be incomplete
• Column information is completely missing
• The symbol names are fully qualified in the first one. I think that's actually kinda nice, but I'm not sure why there is a difference.
• The line number of the second frame is completely wrong

As a reminder, it is our intent to make this backend the default for debug mode, so it's actually even more important for debug info to be good in this backend than in the llvm backend!