[kernel] Expose a sysconf page to userspace

A structure, system_config, which is dynamically defined by the
definitions/sysconf.yaml config, is now mapped into every user address
space. The kernel fills this with information about itself and the
running machine.

User programs access this through the new j6_sysconf fake syscall in
libj6.

See: Github bug #242
See: [frobozz blog post](https://jsix.dev/posts/frobozz/)

Tags:

src/libraries/j6/j6.module

@@ -6,10 +6,15 @@ j6 = module("j6",
     sources = [
         "init.cpp",
         "include/j6/syscalls.h.cog",
+        "include/j6/sysconf.h.cog",
         "syscalls.s.cog",
+        "sysconf.cpp.cog",
     ])
 
 from glob import glob
+from os.path import join
+
+sysconf = join(source_root, "definitions/sysconf.yaml")
 definitions = glob('definitions/**/*.def', recursive=True)
 
 j6.add_depends([
@@ -17,3 +22,8 @@ j6.add_depends([
         "syscalls.s.cog",
     ], definitions)
 
+j6.add_depends([
+        "include/j6/sysconf.h.cog",
+        "sysconf.cpp.cog",
+    ], [sysconf])
+