Fix #7824: failure to build a cross-compiler on Haiku.

* force creation of a cross-compiler for both gcc2 and gcc4 when
  building on Haiku (by suffixing the build and host machine with
  '_buildhost')
Remove whole sys-include folder when cross-compiler has been built.

* at least for gcc2, we used to leave the 'os' subfolder in there,
  which may have caused problems when Haiku's headers have changed
  since the last time the compiler was built.