y2038: fix: Add missing libc_hidden_def attribute for some syscall wrappers

During the conversion to support 64 bit time on some architectures with
__WORDSIZE == 32 && __TIMESIZE != 64 the libc_hidden_def attribute for
eligible functions was by mistake omitted.

This patch fixes this issue and exports (and allows using) those
functions when Y2038 support is enabled in glibc.