s390: Be consistent with emulation warnings about unsupported
rounding modes in absence of the floating-point extension facility.
For some insns we would vassert for others we'd give a warning.
Now we always issue an emulation warning.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2681 8f6e269a-dfd6-0310-a8e1-e2731360e62c