From: "Martin J. Bligh" <mbligh@aracnet.com>

Patch from Martin Bligh, based on observations by Andrew Theurer and Bill
Irwin.

TARGET_CPUS is used as a cpu mask by some things, and an apic mask for
others.  For SMP, that doesn't matter (they're the same), but for Summit it
does.  This patch changes TARGET_CPUS to consistently be a cpu mask
everywhere.  Should be a no-op for normal platforms.

Invalid arguments to cpu_mask_to_apicid for clustered apic mode
architectures will now return the broadcast apicid, in order to ensure
someone still gets the interrupt (was the default init value, and is
safest).