s390x: Implement SAM{24,31,64}

The SAM instructions simply change 2 bits in PSW.MASK to advertise
the current memory mode. While we can't fully guarantee that 31 bit
mode (or even remotely 24 bit mode) actually work correctly, we don't
check whether lpswe modifies these bits, so we shouldn't keep the
guest from executing SAM instructions either.

This patch implements all SAM instrutions with their actual PSW changing
semantics, making more recent Linux kernels boot properly which do issue
a SAM31 call during early boot.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>
