Add Dreamcast 32MB RAM Mod

[cepawiel]

This change adds support for the Dreamcast 32MB Mod as detailed here: tsowell's original blog post and
32MB Mod Wiki Page

Image showing new setting:

Support was added to KallistiOS (Open Source SDK for Dreamcast/NAOMI) for detection and usage of the 32MB modification in Janurary.

Tested using the KOS memtest32 example program.

32MB Mod Disabled:

This console has 16777216 bytes of system memory,
 with top of memory located at 0x8d000000.

Beginning memtest routine...
 Base address: 0x8c100000
 Number of bytes to test: 15663104
  memTestDataBus: PASS
  memTestAddressBus: PASS
  memTestDevice: PASS
Test passed!

32MB Mod Enabled:

This console has 33554432 bytes of system memory,
 with top of memory located at 0x8e000000.

Beginning memtest routine...
 Base address: 0x8c100000
 Number of bytes to test: 32440320
  memTestDataBus: PASS
  memTestAddressBus: PASS
  memTestDevice: PASS
Test passed!

[gyrovorbis]

Just wanted to chime in as a developer who has the 32MB mod that this would be fantastic to have. :)

[cepawiel]

• Changed PR destination branch to dev
  - Rebased changes on existing dev branch I did something incorrect with my rebase, on it now
• Added config option for libretro builds

[cepawiel]

Fixed previous rebase error

[flyinghead]

Thanks for the changes.

There's another issue regarding savestates:
Loading a 16MB savestate into a 32MB environment, or vice versa, will cause a crash.
I suggest to create a new savestate version (see serializer.h) and save a 16MB/32MB flag right after the savestate version (see Serializer and Deserializer constructors.) When loading the savestate an exception should be thrown if the savestate is incompatible with the current ram setting. (This check to be done only for dreamcast savestates, not arcade ones)

[flyinghead]

Not critical but nice to have: the Dreamcast 32MB RAM Mod option should be disabled/greyed out when a game is started, since it can't be changed at runtime. (There are already a number of options with this behavior.)

[cepawiel]

I added a check to the Deserialization that throws an exception if the current ram setting does not match the save state and disabled changing the setting while a game is started. Let me know if there's anything else I can fix 😄

[flyinghead]

Looks good to me. I fixed the failing test and rebased on top of the current dev branch.

I'll merge when checks are finished.