linux-user: enable sigaltstack for all architectures There is no reason to limit sigaltstack syscall to just a few architectures and pretend it is not implemented for others. If some architecture is not ready for this, that architecture should be fixed instead. This fixes LP#1516408. Signed-off-by: Michael Tokarev <mjt@tls.msk.ru> Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Riku Voipio <riku.voipio@linaro.org>