RISC-V: correct FCVT.Q.L[U]

RISC-V: correct FCVT.Q.L[U]

While the spec isn't explicit about this, it pointing out the similarity
with the D extension ought to extend to the ignoring of a meaningless
rounding mode: "Note FCVT.D.W[U] always produces an exact result and is
unaffected by rounding mode." Hence the chosen encodings also ought to
match.

Note that to avoid breaking existing code the forms with a 3rd operand
are not removed, which means there continues to be a difference to
FCVT.D.W[U].