backpatch "jit: Add support for LLVM 12."

As there haven't been problem on the buildfarm due to this change,
backpatch 6c57f2ed16e now.

Author: Andres Freund
Discussion: https://postgr.es/m/20201016011244.pmyvr3ee2gbzplq4@alap3.anarazel.de
Backpatch: 11-, where jit support was added

Author: anarazel