[flang][runtime] Don't round hexadecimal floating-point input (#76586)

Fortran 2023 subclause 13.7.2.3.8 discusses input rounding only in the
context of decimal-to-binary conversion. There is no mention of rounding
for hexadecimal floating-point input conversion. At least one Fortran
compiler seems to have interpreted this silence as implying no rounding.
(Note that this is not the same thing as rounding to zero (RZ), which
would return +/-HUGE() for overflow.)