posix: sys/time.h: Add workaround for outdated newlib used by Xtensa

Unfortunately, Zephyr SDK 0.10.0 ships with outdate Newlib 2.0.0
(from 2015 or earlier) which lacks sys/_timeval.h header, requiring
ugly workaround of defining struct timeval inline (the whole idea
was to standardize on sys/_timeval.h header for different libc's).

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