[efi] Provide a multiprocessor API for EFI

[efi] Provide a multiprocessor API for EFI

Provide an implementation of the iPXE multiprocessor API for EFI,
based on using EFI_MP_SERVICES to start up a wrapper function on all
application processors.

Note that the processor numbers used by EFI_MP_SERVICES are opaque
integers that bear no relation to the underlying CPU identity
(e.g. the APIC ID), and so we must rely on our own (architecture-
specific) implementation to determine the relevant CPU identifiers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>