[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:

definitions/sysconf.yaml

@@ -0,0 +1,34 @@
+---
+address: 0x6a360000
+vars:
+  - name: version_major
+    section: kernel
+    type: uint8_t
+
+  - name: version_minor
+    section: kernel
+    type: uint8_t
+
+  - name: version_patch
+    section: kernel
+    type: uint16_t
+
+  - name: version_gitsha
+    section: kernel
+    type: uint32_t
+
+  - name: page_size
+    section: sys
+    type: size_t
+
+  - name: large_page_size
+    section: sys
+    type: size_t
+
+  - name: huge_page_size
+    section: sys
+    type: size_t
+
+  - name: num_cpus
+    section: sys
+    type: uint32_t