bootstrap for 12.0 available here http://mikael.urankar.free.fr/FreeBSD/powerpc64/ghc-8.6.3-boot-powerpc64-freebsd.tar.xz
Patches come from https://gitlab.haskell.org/trommler/ghc/commits/wip/T15916
Most of them are already commited upstream.
these 2 patches are not yet committed:
https://ghc.haskell.org/trac/ghc/ticket/15411 (Disable-unboxed-arrays.patch)
https://gitlab.haskell.org/trommler/ghc/commit/19731a77ed203870f76a53eaf01758efbb5144d3
For the record, upstream PR for FreeBSD/ppc64: https://ghc.haskell.org/trac/ghc/ticket/15916
todo:
backport the patch to older version (or restrict powerpc64 to 8.6.3 only)?
add the missing bits for the 'llvm version' (work/ghc-8.6.3/llvm-targets)?
...For the record, upstream PR for FreeBSD/ppc64: https://ghc.haskell.org/trac/ghc/ticket/15916