Kernel: Implement Page Attribute Table (PAT) support and Write-Combine

This allows us to enable Write-Combine on e.g. framebuffers,
significantly improving performance on bare metal.

To keep things simple we right now only use one of up to three bits
(bit 7 in the PTE), which maps to the PA4 entry in the PAT MSR, which
we set to the Write-Combine mode on each CPU at boot time.

Kernel/Arch/x86/CPUID.h

@@ -58,6 +58,7 @@ enum class CPUFeature : u32 {
     FXSR = (1 << 23),
     LM = (1 << 24),
     HYPERVISOR = (1 << 25),
+    PAT = (1 << 26),
 };
 
 }