x86/boot: Provide PIC aliases for 5-level paging related constants

Provide PIC aliases for the global variables related to 5-level paging, so
that the startup code can access them in order to populate the kernel page
tables.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-35-ardb+git@google.com