x86/boot/compressed: Move startup32_check_sev_cbit() out of head_64.S

Now that the startup32_check_sev_cbit() routine can execute from
anywhere and behaves like an ordinary function, it can be moved where it
belongs.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-15-ardb@kernel.org