Note: We have to check what kind of other 64 bit architectures we have to add.

- The formely pattern used $pkg-$ver, which turned out to be
  a bit too strict.

- After changing the default behavior for 64bit target builds from
  "multilib-by-default" to optional, it is more wise to move the
  option out of the "Expert Features" section to be more visible.

- After finishing the first buildloop we have to go back to
  toplevel srcdir
- This commit also changes the priority of the preconf hook we
  use while building multilib.

  Warning:
  The change of the priority is very sensitive, so you should
  just change it if you really know what you are doing!

- To be able to build "pure" x86-64 systems (non-multilib) we need to remove
  this hardwired multilib by default behavior from the x86-64 architecture
  configuration.

Note:
From now on we do not rely on the glibc32 package anymore by using a second
buildloop while building a 64bit multilib target.