[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

src/include/kernel_args.h

@@ -22,6 +22,7 @@ enum class mod_type : uint32_t {
 	initrd,
 
 	memory_map,
+	page_tables,
 	framebuffer,
 
 	max
@@ -72,7 +73,10 @@ struct header {
 
 	uint8_t _reserved0;
 
-	uint32_t _reserved1;
+	void *pml4;
+	void *page_table_cache;
+	uint32_t num_free_tables;
+
 	uint32_t num_modules;
 	module *modules;