[$] SFrame-based stack unwinding for the kernel

The kernel's perf
events subsystem can produce high-quality profiles, with full
function-call chains, of resource usage
within the kernel itself. Developers, however, often would like to see
profiles of the whole system in one integrated report with, for example,
call-stack information that crosses the boundary between the kernel and
user space. Support for unwinding user-space call stacks in the perf
events subsystem is currently inefficient at best. A long-running effort
to provide reliable, user-space call-stack unwinding within the kernel,
which will improve that situation considerably, appears to be reaching
fruition.