y2038: linux: Provide __timespec_get64 implementation

This patch provides new instance of Linux specific timespec_get.c file placed
in ./sysdeps/unix/sysv/linux/.

When compared to this file version from ./time directory, it provides
__timespec_get64 explicit 64 bit function for getting 64 bit time in the
struct __timespec64 (for compilation using C11 standard).
Moreover, a 32 bit version - __timespec_get internally uses
__timespec_get64.

The __timespec_get is now supposed to be used on systems still supporting 32
bit time (__TIMESIZE != 64) - hence the necessary conversion to 32 bit struct
timespec.

Internally the timespec_get uses __clock_gettime64. This patch is necessary
for having architectures with __WORDSIZE == 32 Y2038 safe.

Build tests:
./src/scripts/build-many-glibcs.py glibcs

Run-time tests:
- Run specific tests on ARM/x86 32bit systems (qemu):
  https://github.com/lmajewski/meta-y2038 and run tests:
  https://github.com/lmajewski/y2038-tests/commits/master



Above tests were performed with Y2038 redirection applied as well as without
to test proper usage of both __timespec_get64 and __timespec_get.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>