Sketch how to load checkpoint into Spike directly

[abejgonzalez]

Simple sketch of how to load a checkpoint into Spike directly (without SpikeTile). This uses hacks to expose the CLINT/PLIC as public members of the sim_t class so it's not expected to compile at first. Also needs to be cleaned + deduped.

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Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

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