s390x: Fix disassembly for various DFP and BFP opcodes (BZ 495817)

s390x: Fix disassembly for various DFP and BFP opcodes  (BZ 495817)

Specifically:
- DFP: add, subtract, divide, multiply
- BFP: load FP integer, load rounded

Change function names from e.g. s390_irgen/emit_LDXBR to
s390_irgen/emit_LDXBRA because LDXBR does not have m3 and m4 operands.
Adjust the returned mnemonic accordingly.

New function s390_format_RRF_UUFF2 for FI[DEX]BRA.

Add adtra_like_disasm to disassemble opcodes for DFP arithmetic.

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