cpucontrol: improve Intel microcode revision check

According to the Intel SDM (Volme 3, 9.11.7) the BIOS signature MSR
should be zeroed before executing cpuid (although in practice it does
not seem to matter).

PR: 192487
Submitted by: Dan Lukes
Reported by: Henrique de Moraes Holschuh
MFC after: 3 days