Adding `lightswitch` as a profile tool

Hi,

Would you be open / interested in adding lightswitch as one of the available profilers? I thought of posting here after lurking in Rust's zulip and seeing some mentions of the issues of dealing with other profilers (cc @bal-e).

The TL;DR is:

• x86_64 and arm64 Linux only, frame pointers not necessary;
• support deeper stacks thanks to not using PERF_SAMPLE_STACK_USER, as of now, no limits in terms of process stack, and up to 200 user + kernel frames;
• various output modes, including to the Firefox profiler, SVG flamegraph, Perfetto, and others;
• written in Rust :)

I think right now the biggest blocker compared to other profilers is that it doesn't accept a command to run and profile, it can either profiler the whole box of the specified pids. This is something I am planning on adding soon and can prioritise if this would be useful.

Start by reviewing rustc-perf's existing profiler integrations and the lightswitch profiler entry points, including src/bpf/profiler.h. Clarify whether lightswitch must first support profiling a command, then define how it should be exposed and how its supported Linux architectures and output modes are handled. Done means lightswitch is available through rustc-perf with the agreed invocation and verified output.