kernel .config lowlatency improvements

[Impact]

The lowlatency kernel in Ubuntu is specifically designed to prioritize high responsiveness, making it ideal for multimedia environments like DAWs and audio processing platforms, as well as soft real-time environments.

With the introduction of a real-time kernel, it might be worth reconsidering the role of the lowlatency kernel and potentially including it as the default kernel in desktop images, focusing on its suitability for desktop-oriented usage.

To achieve this, we can enable additional configuration settings and make it more focused for a low-latency and highly responsive desktop environment.

Optionally (for the future) provide also an additional user-space package that would enable specific run-time kernel settings focused at certain preset workload profiles (e.g, web navigation, gaming, audio processing, etc.).

[Test case]

Use linux-lowlatency in a desktop environment and measure responsiveness of interactive applications.

[Fix]

Enable the following additional .config settings to make this kernel more suitable for a low-latency desktop kernel:

 - CONFIG_NO_HZ_FULL=y: enable access to "Full tickless mode" (shutdown clock tick when possible across all the enabled CPUs if they are either idle or running 1 task - reduce kernel jitter of running tasks due to the periodic clock tick, must be enabled at boot time passing `nohz_full=<cpu_list>`)

 - CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y: move RCU callbacks from softirq context to kthread context (reduce time spent in softirqs with preemption disabled to improve the overall system responsiveness, at the cost of introducing a potential performance penalty, because RCU callbacks are not processed by kernel threads)

 - CONFIG_RCU_LAZY=y: batch RCU callbacks and then flush them after a timed delay instead of executing them immediately (can provide 5~10% power-savings for idle or lightly-loaded systems, this is extremely useful for laptops / portable devices - https://<email address hidden>/)

[Regression potential]

Enabling all these settings can introduce a potential performance regression, but the kernel should result more responsive and make it more suitable for a desktop/multimedia/gaming/audio processing context.