Kernel/riscv64: Add MMU initialization code

We initialize the MMU by first setting up the page tables for the
kernel image and the initial kernel stack.
Then we jump to a identity mapped page which makes the newly created
kernel root page table active by setting `satp` and then jumps to
`init`.

Kernel/Arch/riscv64/pre_init.cpp

@@ -51,9 +51,7 @@ extern "C" [[noreturn]] UNMAP_AFTER_INIT void pre_init(FlatPtr mhartid, Physical
     // Catch traps in pre_init
     RISCV64::CSR::write(RISCV64::CSR::Address::STVEC, bit_cast<FlatPtr>(&early_trap_handler));
 
-    // FIXME: Implement this
-
-    Processor::halt();
+    Memory::init_page_tables_and_jump_to_init();
 }
 
 }