cross-compiling in OpenJDK/IcedTea

- merge cross-compile support patches from OpenEmbedded into IcedRea
- adjust IcedTea build env that it can fully deal with a cross-compilation build
- work out an accepted (by IcedTea and hopefully OpenJDK) way to override and set:
  * compiler, flags and target machine
- rework a part of the OpenJDK build system where it uses a native C-program to read sizes of data structures and generates Java source code out of this
  * got an implementation suggestion by Christian Thalinger (former CacaoVM, today Oracle developer)
  * consensus with Oracle devs needed here; if they prefer different (equally good) solution, implement that
- port all changes mentioned so far to IcedTea7 (Java7 branch)
- depending on interest: merge changes back into OpenJDK (would suggest this only for Java7; could be a time consuming task)

benefit:
 * cross-compiling OpenJDK in an easy way for OE, opens door for EmDebian, OpenWRT, ...
 * when Java7 is released a cross-compiled variant will be available as well
 * Java means long support cycles; integrating the patches lowers the maintenance burden

As the person who implemented the cross-compiling support in OpenEmbedded I can easily add more background information to this blueprint if needed.