[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/kernel-debug.yaml

@@ -23,7 +23,8 @@ variables:
     "-ggdb",
 
     "-D__ELF__",
-    "-D__JSIX__",
+    "-D__jsix__",
+    "-D__j6kernel",
     "-U__linux",
     "-U__linux__",
     "-DPRINTF_ALIAS_STANDARD_FUNCTION_NAMES=1",