I am not sure if we still need the old packages, nothing explicitly
depends on polyml56 or polyml57 according to a grep, not sure if
external packages might (hol and isabelle depend on polyml, the latest
version).
New libffi doesn't have FFI_SYSV for x86/64 unix, this pulls in the
commit for the upstream version which fixes it, and ports that patch to
the 5.7 version. The 5.6 version is unchanged.
For ZHF: #80379
