Add VEX support for the s390x "compare and signal" instructions KEBR,
KDBR, KXBR, KEB, and KDB. For now, let them behave exactly like their
non-signalling counterparts. Enhance the bfp-4 test case to cover these
instructions as well. Update the list of supported instructions in
s390-opcodes.csv. Add a disclaimer to README.s390, explaining that FP
signalling is not handled accurately on s390x at the moment.