[RISCV] Always use signed APSInt in getExactInteger. (#84070)

We were setting based on whether the FP value is positive/negative, but
we really want to know whether the resulting integer will be treated as
a signed or unsigned value. Since we use SINT_TO_FP to convert the
integer to FP, we should always used signed here.

Without this we convert +2147483648.0 to an integer 0x80000000 and
convert it using sint_to_fp which produces -2147483648.0.