arch-spec, pegasus: Enable CPUs to get their cpuid by asking the MMU
os: use MMU-supplied cpuid to find the percpu data area address

This is necessary so that interrupt handlers can find it; the old "let the
 CPUs work out cpuid assignment between themselves" requires that each CPU
 remember its ID (or &percpu_struct), which we can't trust user code to do.
Move os/ subdirectory to pegbox/ in accordance with the Big Rename