Introduce stage0 into the bootstrap process.

* This stage builds the gcc packages to get the shared syslibs, which
  only requires the Haiku glue code.
* Add separate declaration section for stage0 packages to
  HaikuPortsCross repository files.
* For the bootstrap_stage0 platform, fall back to the gcc headers
  provided by the cross-compiler.

build/jam/repositories/HaikuPortsCross/m68k

@@ -4,6 +4,8 @@ BootstrapPackageRepository HaikuPortsCross
 	# architecture "any" packages
 	haikuporter-0-1
 	:
+	# repository architecture packages (stage 0)
+	:
 	# repository architecture packages (stage 1)
 	zlib_bootstrap-1.2.8-2
 	zlib_bootstrap_devel-1.2.8-2