Merge PPC port

	Merge PPC port
	... new directories src/compiler/ppc, src/assembly/ppc
	... other new files
	... new clause in genesis for PPC fixups
	... new files in runtime, PPC conditionals added in other .[ch] files

	Small Makefile cleanups in runtime
	... actually _use_ the dependency information
	... regenerate depends on source changes

	We don't actually use sigreturn() in _any_ present port: conditionals
	changed to make this obvious