x86/sev: Provide PIC aliases for SEV related data objects

Provide PIC aliases for data objects that are shared between the SEV startup
code and the SEV code that executes later. This is needed so that the confined
startup code is permitted to access them.

This requires some of these variables to be moved into a source file that is
not part of the startup code, as the PIC alias is already implied, and
exporting variables in the opposite direction is not supported.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-36-ardb+git@google.com