Solve the y2038 problem for powerpc

AKA Make time_t 64 bits on powerpc(32).

PowerPC currently (until now) was one of two architectures with a 32-bit time_t
on 32-bit archs (the other being i386). This is an ABI breakage, so all ports,
and all local binaries, *must* be recompiled.

Tested by: andreast, others
MFC after: Never
Relnotes: Yes