This change switches the xar package from unmaintained fork of the
original project to the Apple Open Source tarball. See also
https://repology.org/project/xar/versions
Since the package is essentially rewritten from scratch, we take an
opportunity and move it to pkgs/by-name/xa/xar (formatted with nixfmt).
We also remove Windows from the supported platforms because even before
this change pkgsCross.mingwW64.xar failed with
xar> configure: error: can not detect the size of your system's uid_t type
5eee6cf40a