Hacks and fixes to enable picolibc CI on MSP430

[keith-packard]

Here's a series of hacks I wrote which allow me to use mspsim to run picolibc tests. There's only one actual fix here -- extending the CG3 -1 value to 20 bits. Everything else just makes the Tyndall emulator capable of working in picolibc CI.

[pjonsson]

All the changes look reasonable from a quick glance, but the source code for MSPSim silently moved into the Cooja repository (https://github.com/contiki-ng/cooja) a while ago. We need to update README.md with that information.

I started writing some explanations, but the MSPSim entry-point is only half-integrated with the Cooja entrypoint. That headache is not yours, can we gradually cherry-pick these patches (or equivalents) into the MSPSim that lives in the Cooja repository?

[keith-packard]

I'm probably going to abandon this series as the binutils-gdb simulator is already integrated into the crosstool-ng toolchain, making that far easier to add to the picolibc CI environment. Please feel free to do whatever you like to these patches.