[RISCV] Add MC support for FCVT.H.Q and FCVT.Q.H (#220176)
According to the
[SPEC](https://github.com/riscv/riscv-isa-manual/blob/e5c0c60fa1fbfcc1d343e314299d15693485f678/src/unpriv/zfh.adoc?plain=1#L77-L82),
`FCVT.Q.H` and `FCVT.H.Q` are available when both the Q and Zfhmin
extensions are present. This commit adds MC support for these
instructions.
