enable configuration of file splitting via THOR_JIT_N_SPLITS env var

LOKI_GENERATE_CPPCODE is supported because that's what is used in the functor cache creation, but setting this variable also enables the creation of cpp files in the loki factories. Thus we need/want an env variable to steer the setting of how many files to create for the JIT procedure that doesn't mess with the loki factories.

That's what this MR introduces via THOR_JIT_N_SPLITS