[scripts] Make j6libc.py into the codegen package

j6libc.py was initially made for libc to generate stdint.h, but it
gained a few things that aren't libc-specific. Move it to a package and
split the int-types-specific code into codegen.int_types.

src/libraries/libc/include/errno.h.cog

@@ -12,9 +12,9 @@
 
 
 /**[[[cog code generation
-from j6libc import arch_includes
+from codegen import arch_includes
 
-arch_includes("errno.h")
+arch_includes("errno.h", root="__j6libc")
 
 ]]]*/
 /*[[[end]]]*/