In x2APIC mode, IPI generation is atomic because it is performed by
single ICR MSR write. This is in contrast with the xAPIC mode, where
we must read current ICR value, do bit fiddling and perform two 32-bit
register writes. As a consequence, there is no need to disable
interrupts around ICR value calculation and write.

Note that typical users of ipi_raw() and ipi_vectored() take spinlock,
which already disables interrupts. For them, the change removes
unneeded CLI and POPFL/Q instructions.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks