y2038: linux: Provide __timerfd_gettime64 implementation

y2038: linux: Provide __timerfd_gettime64 implementation

This patch replaces auto generated wrapper (as described in
sysdeps/unix/sysv/linux/syscalls.list) for timerfd_gettime with one which
adds extra support for reading 64 bit time values on machines with
__TIMESIZE != 64.
There is no functional change for architectures already supporting 64 bit
time ABI.

This patch is conceptually identical to timer_gettime conversion already
done in sysdeps/unix/sysv/linux/timer_gettime.c.
Please refer to corresponding commit message for detailed description of
introduced functions and the testing procedure.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
---
Changes for v4:
- Update date from 2019 to 2020

Changes for v3:
- Add missing libc_hidden_def()

Changes for v2:
- Remove "Contributed by" from the file header
- Remove early check for (fd < 0) in __timerfd_gettime64 as the fd
  correctness check is already done in Linux kernel
- Add single descriptive comment line to provide concise explanation
  of the code