linux-user: add gcov support to preexit_cleanup

linux-user: add gcov support to preexit_cleanup

As we don't always take the normal exit path when running a guest we
can skip the normal exit destructors where gcov normally dumps it's
info. The GCC manual suggests long running programs use __gcov_dump()
to flush out the coverage state periodically so we use that here.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>