Add thumb2 mrc il

[jonathanzetier]

This PR:

• Updates an outdated README (build instructions seem to be from the era when the arch was a separate repo).
• Makes a minor change to disassembly: using the same rX for register names as everywhere else did in MRC disassembly rather than RX, which makes it possible to see the source of register values when a register is highlighted.
• Adds IL to the MRC/MRRC/MCR/MCRR (and *2 variants) in thumb mode. Nothing too fancy here, the intrinsics were already implemented for ARM mode, so this was just bringing them over to thumb mode too.

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[galenbwill]

Thanks for your submission. I will review this after the 5.0 release is done.