x86/coco: Export cc_vendor

It will be used in different checks in future changes. Export it directly
and provide accessor functions and stubs so this can be used in general
code when CONFIG_ARCH_HAS_CC_PLATFORM is not set.

No functional changes.

[ tglx: Add accessor functions ]

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230318115634.9392-2-bp@alien8.de