x86: Extend validity of bsp_to_cpu

As we hard-wire the BSP to CPU 0 anyway and cpuid_apic_id equals
cpu_index, bsp_to_cpu can also be based on the latter directly. This
will help an early user of it: KVM while initializing mp_state.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
