plat/kvm/x86: Make SMP init code resolve its own `start16` relocations

plat/kvm/x86: Make SMP init code resolve its own `start16` relocations

Before waking up the secondary cores, the SMP initialization code copies
the, now position independent through `uk_reloc`, 16-bit and 32-bit
bootstrapping code to a physical page, in the lower 1 Mib of physical
memory. Since, at this point of execution, the immediate values used by
this bootstrapping code take the form of `UK_RELOC_PLACEHOLDER`, they
will need to be properly resolved after being moved into lower memory.

Therefore, add a new locally defined `struct uk_reloc` array holding the
hardcoded corresponding entries of these relocations, adapted to
reference the desired relocation address. Use this array after memory
copying the bootstrapping code to lower memory to resolve its
corresponding `start16` relocations.

Encode these entries with the help of `start16` related macro's and
since these definitions are starting to visually occupy a lot of space,
move everything to `start16_helpers.h`, a separate header file.

Signed-off-by: Sergiu Moga <sergiu.moga@protonmail.com>
Reviewed-by: Dragos Petre <dragos.petre27@gmail.com>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #772