uprobe-stress: add uprobe stress-testing tool

[anakryiko]

Implemented a stress-testing tool for uprobes/uretprobes.

It constantly triggers a set of user space functions, in parallel it attached and detached uprobes and uretprobes to random subset of them. To make things more interesting we also randomly and in parallel mmap() /proc/self/exe and fork() process, to trigger all the different code paths in uprobe-related functionality in the kernel.