Local APIC timer

[IsaacWoods]

I am planning to use the local APIC timer for a per-CPU scheduler timer to pre-empt greedy threads, so we'd ideally always like to be able to configure it correctly. We therefore need multiple back-ups to correctly find its frequency:

• [x] Check if we're running under a hypervisor and if it gives us the frequency
• [x] Get the core crystal clock frequency
• [ ] Provide a suitable backup choice on known microarchitectures
• [ ] Use another timer (such as the ACPI timer, or worst case the PIT) to time it for a known period of time

This suggests the KVM has special support for giving us the APIC frequency. This also suggests that linux has a dedicated CPUID leaf for the hypervisor to specify timer frequencies

[IsaacWoods]

Note: for KVM to publish the 0x4000_0010 leaf, we need to add the vmware-cpuid-freq and invtsc cpu features