main/postgresql: split llvm support
llvm-libs is about 60 MB installed, plus bitcode which is another 19 MB. I'm going to hazard a guess that most postgres users don't need it.
https://github.com/postgres/postgres/blob/master/src/backend/jit/README:
To avoid the main PostgreSQL binary directly depending on LLVM, which would prevent LLVM support being independently installed by OS package managers, the LLVM dependent code is located in a shared library that is loaded on-demand.