After a chat on #gentoo-haskell, it turns out that compiling on ramfs and Hardened do not go together.
ramfs does not support XATTRs (tmpfs does), which paxctl-ng uses to store PaX markings, so you end up with a GHC that has no -m (no MPROTECT, i.e. no W^X) marking set. And one of the very first things GHC does is a RWX mmap()...