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.
This isn't particularly useful because by the time we've entered
init() the CPU had better support x86_64 anyway. However this shows the
CPU flag in System Monitor - even in 32-bit mode.
