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