* refactor: add new module for backend-specific code
* replace ifdefs on rwkv_v3
* replace ifdefs on llama_v2
* replace ifdefs on llama_v3
* replace ifdefs on gpt2_v3
* replace ifdefs on gptj_v3
* replace ifdefs on mpt_v3
* replace ifdefs on neox_v3
* replace ifdefs on whisper
* adjust build for kcpp_backend
Mostly a:
sed 's/ gpttype_adapter\([^ ]*\)\.o / gpttype_adapter_default.o kcpp_backend\1.o /g' Makefile
and include variant objects for kcpp_backend.