x86/boot: Annotate the Real Mode entry points

x86/boot: Annotate the Real Mode entry points

... because its already hard enough to follow.  Cross reference the locations
in C which set the entrypoints up, and state the alignment requirements and
entry conditions.

Drop a redundant .align 16, and panic() in do_boot_cpu() if the AP trampoline
isn't set up properly rather than blindly continuing and letting the APs
execute junk, or shifting part of the address into unrelated fields in ICR.

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