Fix IBRS for machines with IBRS_ALL capability.

When turning IBRS mitigation using sysctl, as opposed to loader tunable,
send IPI to tweak MSR on all cores. Right now code only performed MSR write
onr the CPU where sysctl was run.

Properly report hw.ibrs_active for IBRS_ALL. Split hw_ibrs_ibpb_active out
from ibrs_active, to keep the current semantic of guiding kernel entry and
exit handlers.

Reported and tested by: mav
Sponsored by: The FreeBSD Foundation
MFC after: 1 week