lib: posix, minlibc: struct itimerspec is defined by sys/timespec.h

Newlib has it defined in sys/timespec.h, and thus per the established
conventions, everything else relies on it being there. Specifically,
minimal libc acquires sys/timespec.h with a similar definition, and
POSIX headers rely on that header. Still with a workaround for old
Newlib version as used by Xtensa (but all infrastructure for that is
already there; actually, this patch removes duplicate similar-infra,
which apparently didn't work as expected by now, so now we have a
single workaround, not 2 different once).

To emphasize a point, now there 2 headers:

sys/_timespec.h, defining struct timespec, and
sys/timespec.h, defining struct itimerspec

That's how Newlib has it, and what we faithfully embrace and follow,
because otherwise, there will be header conflicts depending on
various libc and POSIX subsys options.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>