Switch to a 64-bit version of the kernel and rt0 code

The switch to 64-bit mode allows us to use 48-bit addressing and to
relocate the kernel to virtual address 0xffff800000000000 + 1M. The
actual kernel is loaded by the bootloader at physical address 1M.

The rt0 code has been split in two parts. The 32-bit part provides the
entrypoint that the bootloader jumps to after loading the kernel. Its
purpose is to make sure that:
- the kernel was booted by a multiboot-compliant bootloader
- the multiboot info structures are copied to a reserved memory block
  where they can be accessed after enabling paging
- the CPU meets the minimum requirements for the kernel (CPUID, SSE,
  support for long-mode)

Since paging is not enabled when the 32-bit code runs, it needs to
translate all memory addresses it accesses to physical memory addresses
by subtracting PAGE_OFFSET. The 32-bit rt0 code will set up a page table
that identity-maps region: 0 to 8M and region: PAGE_OFFSET to
PAGE_OFFSET+8M. This ensures that when paging gets enabled, we will still
be able to access the kernel using both physical and virtual memory
addresses. After enabling paging, the 32-bit rt0 will jump to a small
64-bit trampoline function that updates the stack pointer to use the
proper virtual address and jumps to the virtual address of the 64-bit
entry point.

The 64-bit entrypoint sets up the minimal g0 structure required by the
go function prologue for stack checks and sets up the FS register to
point to it. The principle is the same as with 32-bit code (a segment
register has the address of a pointer to the active g) with the
difference that in 64-bit mode, the FS register is used instead of GS
and that in order to set its value we need to write to a MSR.

kernel/kmain.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/achilleasa/gopher-os/kernel/hal"
 	"github.com/achilleasa/gopher-os/kernel/hal/multiboot"
+	"github.com/achilleasa/gopher-os/kernel/kfmt/early"
 )
 
 // Kmain is the only Go symbol that is visible (exported) from the rt0 initialization
@@ -18,10 +19,15 @@ import (
 // Kmain is not expected to return. If it does, the rt0 code will halt the CPU.
 //
 //go:noinline
-func Kmain(multibootInfoPtr uint32) {
-	multiboot.SetInfoPtr(uintptr(multibootInfoPtr))
+func Kmain(multibootInfoPtr uintptr) {
+	multiboot.SetInfoPtr(multibootInfoPtr)
 
 	// Initialize and clear the terminal
 	hal.InitTerminal()
 	hal.ActiveTerminal.Clear()
+	early.Printf("Starting gopher-os\n")
+
+	// Prevent Kmain from returning
+	for {
+	}
 }