Allow disabling specific JITs via CMake Arguments

[PhilippvK]

Last time I've tried building ETISS on MacOS, I had to manually comment out

etiss/JITImpl/CMakeLists.txt

Line 39 in 40c0b92

ADD_SUBDIRECTORY(TCC)

to fix build errors with TinyCC.

This might be related to my System (since CI build seems to work fine), however the proposed feature is still useful for other use-cases, i.e. to strip down compile times and file sizes.

I would just add IF(${ENABLE_TCCJIT}}, IF(${ENABLE_GCCJIT}}, IF(${ENABLE_LLVMJIT}} to that file since the JIT's are hardcoded anyways in https://github.com/tum-ei-eda/etiss/blob/master/JITImpl/CMakeLists.txt.

Alternative argument naming would be ENABLE_JIT_TCC etc., let me know what you would prefer.