stackcount: Resolve stacks using a StackTrace table

stackcount: Resolve stacks using a StackTrace table

The changes for this script are minimal: the inline C probe has been
_significantly_ simplified, and should now perform better since the
stack walk happens in native code inside the kernel thanks to the
BPF_STACK_TRACE table.

The output of the tool should be virtually identical, so the man page
and included examples have essentially no changes.