arm: pmu: Print counter values as hexadecimals

The arm/pmu test prints the value of counters as %ld.  Most tests start
with counters around 0 or UINT_MAX, so having something like -16 instead of
0xffff_fff0 is not very useful.

Report counter values as hexadecimals.

Reported-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>