[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.

src/kernel/kernel.module

@@ -6,7 +6,7 @@ kernel = module("kernel",
     output = "jsix.elf",
     targets = [ "kernel" ],
     description = "jsix kernel",
-    deps = [ "util", "cpu", "bootproto" ],
+    deps = [ "util", "cpu", "bootproto", "j6" ],
     includes = [ "." ],
     sources = [
         "apic.cpp",