Use Zephyr's CMake extension to correctly build and link reactor-c

[erlingrj]

See lf-lang/lingua-franca#2167

[erlingrj]

@petervdonovan would you mind taking a look at this since I believe you authored the CMake build system? There are two changes:

1. I am changing the library name from core to reactor-c to avoid a naming collision with an ESP32 library called libcore.
2. I am moving some Zephyr conditional building into this CMake lists. To make it work I would like to know if we are targeting Zephyr before we define the target (because for Zephyr we must use their Cmake extension). So thus, I gather all the sources in a list (you previously did this for debug printing) and add it to the target sources at the end. I also make a list for compile defs, currently it is only used for defining a PLATFORM_ZEPHYR. PLATFORM_NRF52 etc.

[petervdonovan]

These changes look good and I have confidence that when the tests are passing this PR will be good enough to merge.

[erlingrj]

Thanks Peter.

I updated the lf-ref back to master, it should work when the corresponding lingua-franca PR is through the merge queue.