x86: Detect feature flags "CLWB" and "PCOMMIT"

"The availability of CLWB instruction is indicated by the presence of
the CPUID feature flag CLWB (bit 24 of the EBX register)."

CLWB is similar to CLFLUSHOPT, except that it is not required to discard
cacheline contents.

"On processors that supports PCOMMIT, PCOMMIT is enumerated through
CPUID (CPUID.7.0.EBX[22]) only when the feature is enabled by BIOS."

PCOMMIT is used to cause store-to-memory operations to become persistent
(protected from power failure).

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