x86/boot: Add a pointer to Confidential Computing blob in bootparams

The previously defined Confidential Computing blob is provided to the
kernel via a setup_data structure or EFI config table entry. Currently,
these are both checked for by boot/compressed kernel to access the CPUID
table address within it for use with SEV-SNP CPUID enforcement.

To also enable that enforcement for the run-time kernel, similar
access to the CPUID table is needed early on while it's still using
the identity-mapped page table set up by boot/compressed, where global
pointers need to be accessed via fixup_pointer().

This isn't much of an issue for accessing setup_data, and the EFI config
table helper code currently used in boot/compressed *could* be used in
this case as well since they both rely on identity-mapping. However, it
has some reliance on EFI helpers/string constants that would need to be
accessed via fixup_pointer(), and fixing it up while making it shareable
between boot/compressed and run-time kernel is fragile ...