pselect, ppoll: add time64 syscall support, decouple 32-bit time_t

pselect, ppoll: add time64 syscall support, decouple 32-bit time_t

time64 syscall is used only if it's the only one defined for the arch,
or if the requested timeout length does not fit in 32 bits. on current
32-bit archs where time_t is a 32-bit type, this makes it statically
unreachable.

on 64-bit archs, there are only superficial changes to the code after
preprocessing. both before and after these changes, these functions
copied their timeout arguments to avoid letting the kernel clobber the
caller's copies. now, the copying also serves to change the type from
userspace timespec to a pair of longs, which makes a difference only
in the 32-bit fallback case, not on 64-bit.