64 bit time_t on riscv32

[Florian Weimer]

* Antonios Salios via Libc-help:

> According to a kernel maintainer, the __USE_TIME_BITS64 macro should be
> set on architectures that use 64-bit time [2], otherwise the kernel
> headers will not be able to pick the correct definition.

__USE_TIME_BITS64 is an internal glibc macro.  It is not used on
architectures which have a 64-bit time_t by default.

Surely the UAPI headers know which time size the architecture uses in
the kernel interface, and can be written arcordingly?

Thanks,
Florian