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>