100% behavior tests passing for the x86_64 backend

Issue close criteria:

```
$ stage3/bin/zig test ../test/behavior.zig -target x86_64-linux
1880 passed; 99 skipped; 0 failed.
$ stage3/bin/zig test ../test/behavior.zig -fno-llvm -fno-lld -target x86_64-linux
1796 passed; 184 skipped; 0 failed.
```

When the second command has a number passing >= to the first command, this issue can be closed.

Related:
* #21172