s390_irgen_EX_SS: add initialisations so as to remove (false positive) warnings from...

s390_irgen_EX_SS: add initialisations so as to remove (false positive) warnings from gcc-7.x.

When compiling guest_s390_toIR.c for a 32-bit target (a configuration in which
it will never be used, but never mind), gcc-7.x notices that sizeof(ss.dec) is
larger than sizeof(ss.bytes), so the initialisation of ss.bytes leaves ss.dec.b2
and ss.dec.d2 uninitialised.  This patch causes both variants to be initialised.
When built for a 64 bit target, the existing initialisation of ss.bytes covers
ss.dec completely, so there is no error.