Adding Main CPU Overclock Core Option

[pjft]

Hi. Apologies for the first PR, out of the blue.

For "improved" emulation of the Cave shooters on the CV1000 driver, it appears to be useful to rely on underclocking the CPU for different games (see links below).

This option exists only in the MAME settings, under "Slider controls", after you enable cheats. It's a bit convoluted, and doesn't allow for per-game settings saving, which makes it hard to use in a cabinet/controller-only/no-keyboard setup.

I took a stab at implementing that option as a core option, and from my testing and on the RetroPie forums it seems to work well.

Granted:

• It's my first time changing the MAME codebase, so I might be breaking something terribly by doing this. I am explicitly going after the machine()->firstcpuclock, and only if (for some reason) that firstcpuisn't NULL. That being said, I don't know enough about other edge (or non-edge) cases that this might break.
• I am assuming that the default overclock is always 100%. I am inferring that from https://github.com/libretro/mame2016-libretro/blob/master/src/frontend/mame/ui/ui.cpp#L1970 (where the default is 1000) and I am setting the clock_scale (same as the slider) which I believe is a percentage of the configured clock for the device: https://github.com/libretro/mame2016-libretro/blob/master/src/emu/device.h#L338 but please correct me if anyone knows whether that is not guaranteed in some games or cases, and I can figure it out differently.
• I created an "update_runtime_variables" because I noticed that many (if not all?) of the core options are set on MAME relaunch as command-line options, so if we'd want to later down the line make some of those changes in real-time that can be used. For instance, I imagine that turning on cheats might be feasible in runtime, or perhaps other options, and they can be added there.
• Finally, if there's anything different in terms of how I created the code as far as libretro or project standards - or, if the change itself is not something that you would approve for the project - please feel free to let me know how to improve it, or just that it's not going to be considered. I'm happy to learn, but also hopefully to contribute in what little I can.

Thanks.

References on CPU underclocking and CV1000 games:

• https://www.reddit.com/r/EmulationOnAndroid/comments/cmnxm2/cave_cv1000_games_finally_full_speed_on_android/
• https://shmups.system11.org/viewtopic.php?f=1&t=64939

[barbudreadmon]

looks fine to me

[pjft]

Thank you for accepting this!

[KMFDManic]

@pjft @barbudreadmon I have opened a Pull Request with Additional Values, which have tested well for me with Deathsmiles and a slew of other games. I was trying to fix up Killer Instinct Arcade awhile back for MAME 2003 Xtreme, my fork off of 2003, which incorporates this same CPU Scale Option! Grant2258 was a massive help, in that respect! Nice to see these Options to help lesser spec hardwares! Keep up the amazing work!