s390x: Fix disassembly for BFP/DFP conversion opcodes (BZ 495817)

s390x: Fix disassembly for BFP/DFP conversion opcodes  (BZ 495817)

Specifically:
- BFP convert from/to fixed
- BFP convert from/to logical
- DFP convert from/to fixed
- DFP convert from/to logical

Change function names from e.g. s390_irgen/emit_CEFBR to
s390_irgen/emit_CEFBRA because CFEBR does not have m3 and m4 operands.
Adjust the returned mnemonic accordingly.

New functions fp_convf_disasm and fp_convt_disasm.

Part of fixing https://bugs.kde.org/show_bug.cgi?id=495817