linuxkpi: Define `cpu_data(cpu)`

linuxkpi: Define `cpu_data(cpu)`

`cpu_data(cpu)` evaluates to a `struct cpuinfo_x86` filled with
attributes of the given CPU number. The CPU number is an index in the
`__cpu_data[]` array with MAXCPU entries. On FreeBSD, we simply
initialize all of them like we do with `boot_cpu_data`.

While here, we add the `x86_model` field to the `struct cpuinfo_x86`. We
use `CPUID_TO_MODEL()` to set it.

At the same time, we fix the value of `x86` which should have been set
to the CPU family. It was using the same implementation as
`CPUID_TO_MODEL()` before. It now uses `CPUID_TO_FAMILY()`.

Reviewed by:	manu
Approved by:	manu
Differential Revision:	https://reviews.freebsd.org/D38542