cross devel hpkg: use suffix/subdir for secondary arch

* Libraries for a secondary architectures must be placed in a respective
  subdir.
* Add a suffix to the package name when building for a secondary
  architecture. Looks a bit weird
  ("haiku_cross_devel_sysroot_x86_gcc_x86.hpkg"), but is consistent.
build_cross_tools_gcc4: move sysroot dir into install dir

This allows us to sneak in system include and library search dirs in
haikuporter without doing anything outside the cross tools dir.