Fortran: [PR93727] Add EX format rounding for truncated hex mantissa

Fortran: [PR93727] Add EX format rounding for truncated hex mantissa

Implement proper rounding of the hex mantissa in write_ex when the
user specifies a d smaller than full precision.  All Fortran ROUND=
modes are supported: ROUND_NEAREST (ties-to-even), ROUND_COMPATIBLE
(ties away from zero), ROUND_UP, ROUND_DOWN, and ROUND_ZERO.
ROUND_PROCDEFINED and ROUND_UNSPECIFIED default to ROUND_NEAREST on
IEEE 754 systems, consistent with the decimal format behaviour.

Carry propagation handles the case where incrementing a string of
trailing F hex digits reaches the integer digit; if that overflows
(F → 16) the output is normalized by setting the integer digit to 8
and incrementing the binary exponent by one.

Assisted by: Claude Sonnet 4.6

	PR fortran/93727

libgfortran/ChangeLog:

	* io/write.c (write_ex): Replace simple truncation with
	rounding-aware logic respecting dtp round_status.  Add carry
	propagation and integer-digit normalization.
	* io/write_float.def: Change use of GFC_UINTEGER_8 to
	long long unsigned.

gcc/testsuite/ChangeLog:

	* gfortran.dg/EXformat_3.F90: New test covering rounding for
	KIND=4, 8, 10, and 16: clear round-up, ties-to-even (truncate
	and round-up cases), carry propagation, and normalization.
	* gfortran.dg/EXrounding.F90: New test checking the various
	rounding modes for all kinds.