[RISCV][MC] Fix all remaining fcvt instructions that didn't accept rounding...

[RISCV][MC] Fix all remaining fcvt instructions that didn't accept rounding mode but should have (#67889)

This is a follow-up to #67555, performing the same fix for the other
instructions that had this issue:
* fcvt.d.w
* fcvt.d.wu
* fcvt.s.h
* fcvt.d.h

As before, we stick to the 'rne' default because this gives maximum
compatibility with older LLVM and GNU tools when disassembling. I've
also double checked disassembling fp-default-rounding-mode.s with GNU
objdump to re-confirm it uses frm=0b000 for these instructions.