[libj6] Move remaining j6 headers out of src/include

This means the kernel now depends on libj6. I've added the macro
definition __j6kernel when building for the kernel target, so I can
remove parts with #ifdefs.

configs/rules.ninja

@@ -37,7 +37,7 @@ rule compile_asm
   deps = gcc
 
 rule parse_cog
-  command = $cog -o $out -d $cogflags $in
+  command = $cog -o $out -d -D target=$target $cogflags $in
   description = Parsing $name
 
 rule exe