System-wide profiling support

[Ism Hong]

First of all, thank you for this amazing work.
It's a nightmare to cross compile perf for arm/aarch64 with libdwarf support.
not-perf is really useful for embeeded Linux profiling and built-in flamegraph generator is also nice feature. :)

Currently, it seems not-perf only support profiling specific process(pid) with -p/-P argument.
I am wondering is it easy to add support for system-wide profiling option, just like the -a/--all-cpus.
Thanks again for this great work.