OpenBSD 7.8 will require OpaquePointer for FILE. (#84478)
OpenBSD 7.8 snapshots hide the type information for FILE. Therefore, the
types for the standard stdio streams should (regrettably) be
OpaquePointer, due to the well-discussed issue of losing type
information for forward-declared C types.
We explicitly drop to void * in the LibcOverlayShims for backwards
compatibility, since OpenBSD 7.8 is not yet released.
