x86: Add a helper for the BSP's final init sequence common to all flavors

Add bsp_rest_init() to dedup bringing up APs and doing SMP initialization
across 32-bit, 64-bit, and EFI flavors of KVM-unit-tests.  The common
bucket will also be used in future to patches to init things that aren't
SMP related and thus don't fit in smp_init(), e.g. PMU setup.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-15-seanjc@google.com