x86/sev: Export startup routines for later use

Create aliases that expose routines that are part of the startup code to
other code in the core kernel, so that they can be called later as well.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-38-ardb+git@google.com