Show AMD SEV bits during identify CPU in dmesg.

Show AMD SEV bits during identify CPU in dmesg.

Enable identifycpu() to discover and show AMD SEV related information
provided by cpuid.
The "crypt bit" for page table entries is stored in amd64_pos_cbit,
although it is not used yet.
Registers ecx and edx provide the number of guest and minimum ASID
for SEV-only guests.  At least the latter value can be configured
in the BIOS, so it is useful to have this information in dmesg.
Therefore define emtpy bit masks for printf("%b") to get the raw
numbers.

from hshoexer@; OK mlarkin@