[boot] Set up initial page tables

Set up initial page tables for both the offset-mapped area and the
loaded kernel code and data.

* Got rid of the `loaded_elf` struct - the loader now runs after the
  initial PML4 is created and maps the ELF sections itself.
* Copied in the `page_table` and `page_table_indices` from the kernel,
  still need to clean this up and extract it into shared code.
* Added `page_table_cache` to the kernel args to pass along free pages
  that can be used for initial page tables.

Tags: paging

modules.yaml

@@ -62,6 +62,7 @@ modules:
             - src/boot/hardware.cpp
             - src/boot/loader.cpp
             - src/boot/memory.cpp
+            - src/boot/paging.cpp
             - src/boot/support.cpp
 
     nulldrv: