[flang] Get ppc64le build bot back up

A recent change assumed that the native C++ "long double" maps to
a Fortran data type; but this turns out to not be true for ppc64le,
which uses "double-double" for "long double".

This is a quick patch to get the ppc64le flang build bot back up.
A better fix that either uses HostTypeExists<> or replaces "long double"
with "ieee128_t" (or some other solution) is expected to follow soon.

Differential Revision: https://reviews.llvm.org/D124423