arch/x86: Unbreak SMP startup on x86_64

A last minute "cleanup" to the EFI startup path (on a system where I
had SMP disabled) moved the load of the x86_cpuboot[0] entry into RBP
into the main startup code, which is wrong because on auxiliary CPUs
that's already set up by the 16/32 bit entry code to point to the
OTHER entries.

Put it back where it belongs.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>