linux-user: Flush out implementation of gettimeofday

linux-user: Flush out implementation of gettimeofday

The first argument, timeval, is allowed to be NULL.

The second argument, timezone, was missing.  While its use is
deprecated, it is still present in the syscall.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200213032223.14643-6-richard.henderson@linaro.org>
[lv: add "#if defined(TARGET_NR_gettimeofday)"]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>