plat/common/x86: Fix `libukreloc` error for non-`PIE` `SMP` builds

plat/common/x86: Fix `libukreloc` error for non-`PIE` `SMP` builds

After the merge of
commit cf8cc65cb0ae ("plat/kvm/x86: Make SMP init code resolve its own `start16` relocations")
SMP builds that were not built with `libukreloc` would fail due to
external references to `x86_start16_*` symbols no longer being declared.

Thus, fix this by making their declaration present regardless of
`CONFIG_LIBUKRELOC` being enabled or not. Furthermore, guard the no
longer necessary `START16_UKRELOC_*` macro's, as they are not needed
if the previously mentioned configuration is not enabled.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Marco Schlumpp <marco@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #1037