Allow running without syscall tracepoints (#2990)

Currently we fail to run if tracepoints can't be set on various sub-events of /sys/kernel/debug/tracing/events/syscalls/ . Currently these are used to detect various offsets when BTF is not available.

Since some kernel configurations don't have syscall tracepoints, but do have BTF info, things might still work if we just keep running here.

I have confirmed that with this patch, the agent starts up and seems to work normally on a kernel without CONFIG_FTRACE_SYSCALLS, which fails without this patch.

main.go

@@ -250,7 +250,7 @@ func mainWithExitCode() flags.ExitCode {
 	}
 
 	if err = tracer.ProbeTracepoint(); err != nil {
-		return flags.Failure("Failed to probe tracepoint: %v", err)
+		log.Warnf("Failed to probe tracepoint: %v. Parca-agent may fail to run on some kernel versions.", err)
 	}
 
 	externalLabels := reporter.Labels{}