linux-user: assume __NR_gettid always exists

The gettid syscall was introduced in Linux 2.4.11. This is old enough
that we can assume it always exists and thus not bother with the
conditional backcompat logic.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20190320161842.13908-2-berrange@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>