hwpmc: events don't seem to distinguish kernel and userland callchains anymore

[Ryan Stone]

Historically when processing callchain events from hwpmc, you could
use PMC_CALLCHAIN_CPUFLAGS_TO_USERMODE(ev..pl_u.pl_cc.pl_cpuflags) to
distinguish callchains that were captured in user mode from those
captured in kernel mode.  However, on 12.0-RELEASE and a month-old
head, I have noticed that this macro never returns true anymore.  Is
there any way to make this distinction now, beyond an
architecture-specific hack based off of the sample address?