libstd: handle rmdirZ INVAL error (#10145)

The INVAL error was marked unreachable which prevents handling
of the error at a higher level.

It seems like it should map to BadPathError based on the man page for
rmdir (and an incomplete understanding of DeleteDirError), which says:

```
EINVAL pathname has .  as last component.
```

lib/std/os.zig

@@ -2538,7 +2538,7 @@ pub fn rmdirZ(dir_path: [*:0]const u8) DeleteDirError!void {
         .PERM => return error.AccessDenied,
         .BUSY => return error.FileBusy,
         .FAULT => unreachable,
-        .INVAL => unreachable,
+        .INVAL => return error.BadPathName,
         .LOOP => return error.SymLinkLoop,
         .NAMETOOLONG => return error.NameTooLong,
         .NOENT => return error.FileNotFound,