x86/CPU/AMD: Track SNP host status with cc_platform_*()

The host SNP worthiness can determined later, after alternatives have
been patched, in snp_rmptable_init() depending on cmdline options like
iommu=pt which is incompatible with SNP, for example.

Which means that one cannot use X86_FEATURE_SEV_SNP and will need to
have a special flag for that control.

Use that newly added CC_ATTR_HOST_SEV_SNP in the appropriate places.

Move kdump_sev_callback() to its rightful place, while at it.

Fixes: 216d106c

 ("x86/sev: Add SEV-SNP host initialization support")
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Tested-by: Srikanth Aithal <sraithal@amd.com>
Link: https://lore.kernel.org/r/20240327154317.29909-6-bp@alien8.de