Kernel: Detect APs and boot them into protected mode

This isn't fully working, the APs pretend like they're
fully initialized and are just halted permanently for now.

Kernel/Arch/i386/CPU.cpp

@@ -46,12 +46,6 @@
 
 namespace Kernel {
 
-struct [[gnu::packed]] DescriptorTablePointer
-{
-    u16 limit;
-    void* address;
-};
-
 static DescriptorTablePointer s_idtr;
 static DescriptorTablePointer s_gdtr;
 static Descriptor s_idt[256];
@@ -391,6 +385,16 @@ void flush_gdt()
         : "memory");
 }
 
+const DescriptorTablePointer& get_gdtr()
+{
+    return s_gdtr;
+}
+
+const DescriptorTablePointer& get_idtr()
+{
+    return s_idtr;
+}
+
 void gdt_init()
 {
     s_gdt_length = 5;
@@ -819,6 +823,14 @@ void cpu_setup()
     }
 }
 
+u32 read_cr0()
+{
+    u32 cr0;
+    asm("movl %%cr0, %%eax"
+        : "=a"(cr0));
+    return cr0;
+}
+
 u32 read_cr3()
 {
     u32 cr3;
@@ -833,6 +845,14 @@ void write_cr3(u32 cr3)
                  : "memory");
 }
 
+u32 read_cr4()
+{
+    u32 cr4;
+    asm("movl %%cr4, %%eax"
+        : "=a"(cr4));
+    return cr4;
+}
+
 u32 read_dr6()
 {
     u32 dr6;