Add definitions for AMD Spectre/Meltdown CPUID information
No functional change, aside from printing recognized bits in CPU
The bits are documented in 111006-B "Indirect Branch Control Extension"
124441 "Speculative Store Bypass Disable."
Notably missing (left as future work):
* Integration with hw.spec_store_bypass_disable and hw_ssb_active flag,
which are currently Intel-specific
* Integration with hw_ibrs_active global flag, which are currently
* SSB_NO integration in hw_ssb_recalculate()
* Bhyve integration (PR 235010)