Add definitions for AMD Spectre/Meltdown CPUID information
No functional change, aside from printing recognized bits in CPU
identification.
The bits are documented in 111006-B "Indirect Branch Control Extension"[1] and
124441 "Speculative Store Bypass Disable."[2]
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
Intel-specific
* SSB_NO integration in hw_ssb_recalculate()
* Bhyve integration (PR 235010)
[1]:
https://developer.amd.com/wp-content/resources/111006-B_AMD64TechnologyIndirectBranchControlExtenstion_WP_7-18Update_FNL.pdf
[2]:
https://developer.amd.com/wp-content/resources/124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf
PR: 235010 (related, but does not fix)
MFC after: a week
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