Kernel: Move kernel above the 3GB virtual address mark

The kernel and its static data structures are no longer identity-mapped
in the bottom 8MB of the address space, but instead move above 3GB.

The first 8MB above 3GB are pseudo-identity-mapped to the bottom 8MB of
the physical address space. But things don't have to stay this way!

Thanks to Jesse who made an earlier attempt at this, it was really easy
to get device drivers working once the page tables were in place! :^)

Fixes #734.

Kernel/Arch/i386/CPU.h

@@ -95,6 +95,8 @@ public:
         m_raw |= value & 0xfffff000;
     }
 
+    void clear() { m_raw = 0; }
+
     u64 raw() const { return m_raw; }
     void copy_from(Badge<PageDirectory>, const PageDirectoryEntry& other) { m_raw = other.m_raw; }
 
@@ -104,6 +106,7 @@ public:
         UserSupervisor = 1 << 2,
         WriteThrough = 1 << 3,
         CacheDisabled = 1 << 4,
+        Huge = 1 << 7,
         Global = 1 << 8,
         NoExecute = 0x8000000000000000ULL,
     };
@@ -114,6 +117,9 @@ public:
     bool is_user_allowed() const { return raw() & UserSupervisor; }
     void set_user_allowed(bool b) { set_bit(UserSupervisor, b); }
 
+    bool is_huge() const { return raw() & Huge; }
+    void set_huge(bool b) { set_bit(Huge, b); }
+
     bool is_writable() const { return raw() & ReadWrite; }
     void set_writable(bool b) { set_bit(ReadWrite, b); }