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')
(cherry picked from commit df69e209bbacd07fdfea9d9efcfc8e1c0dfedaa0)

Conflicts:
	build/scripts/build_cross_tools_gcc4
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.
(cherry picked from commit 92bb2fb33e2d0f4aa766be095057fe63ff17bc8e)