sigtimedwait: add time64 syscall support, decouple 32-bit time_t

sigtimedwait: 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. on current 32-bit archs, the timeout is passed via an
intermediate copy to remove the assumption that time_t is a 32-bit
type.