openhcl/tdx: kernel and openhcl_boot spec compliance for AP mailbox protocol

[chris-oo]

The assembly stub in openhcl_boot for the AP mailbox protocol assumes that the OS will not use the ReservedForOs section (https://uefi.org/htmlspecs/ACPI_Spec_6_4_html/05_ACPI_Software_Programming_Model/ACPI_Software_Programming_Model.html#multiprocessor-wakeup-mailbox-structure). We should instead fix the assembly stub to not use that section.

Additionally, investigate if the kernel can also not rely on firmware mapping more than just the mailbox page as identity mapped.