x86/cpu: Drop cpu_devs[] and $VENDOR_init_cpu() hooks

x86/cpu: Drop cpu_devs[] and $VENDOR_init_cpu() hooks

These helpers each fill in a single cpu_devs[] pointer, and since c/s
00b4f4d0f "x86/cpuid: Drop get_cpu_vendor() completely", this array is read
exactly once on boot.

Delete the hooks and cpu_devs[], and have early_cpu_detect() pick the
appropriate cpu_dev structure directly.

As early_cpu_init() is empty now other than a call to early_cpu_detect(), and
this isn't expected to change moving forwards, rename the latter and delete
the former.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>