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Add RPI4 64 + N2 + OGA Support

Add optional automatic frame skipping

This PR uses the `RETRO_ENVIRONMENT_SET_AUDIO_BUFFER_STATUS_CALLBACK`
environment to implement optional automatic frame skipping, based on
the implementation in snes9x2005.

[3DS] Fix crashes when loading savestates

The nonportable LE macros in Blargg SPU can allow unaligned loads,
which cause ARM exceptions on 3DS.

The portable code avoids these problems, and generates good enough
assembly, so it should be used instead.

Minor fixes

REGISTER_2180 is used as the body of an if-statement with no curly
braces, so only the second statement executes regardless of whether
the if-expression evaluated to true.

To fix this, use the standard technique of adding
`do { ... } while (0)` so that the macro can be used like a regular
statement.

These functions are only used conditionally on SNES_SUPPORT_MULTI_CART,
so their definitions should also be conditional to prevent unused
function warnings.

msvc2010: Buildfix