dfp, real: Fix up FLOAT_EXPR/FIX_TRUNC_EXPR constant folding between dfp and large...

dfp, real: Fix up FLOAT_EXPR/FIX_TRUNC_EXPR constant folding between dfp and large _BitInt [PR120631]

The following testcase shows that while at runtime we handle conversions
between _Decimal{64,128} and large _BitInt correctly, at compile time we
mishandle them in both directions, in one direction we end up in ICE in
decimal_from_integer callee because the char buffer is too short for the
needed number of decimal digits, in the conversion of dfp to large _BitInt
we return 0 in the wide_int.

The following patch fixes the ICE by using larger buffer (XALLOCAVEC
allocated, it will be never larger than 65536 / 3 bytes) in the larger
_BitInt case, and the other direction by setting exponent to exp % 19
and instead multiplying the result by needed powers of 10^19 (10^19 chosen
as largest power of ten that can fit into UHWI).

2025-06-18  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/120631
	* real.cc (decimal_from_integer): Add digits argument, if larger than
	256, use XALLOCAVEC allocated buffer.
	(real_from_integer): Pass val_in's precision divided by 3 to
	decimal_from_integer.
	* dfp.cc (decimal_real_to_integer): For precision > 128 if finite
	and exponent is large, decrease exponent and multiply resulting
	wide_int by powers of 10^19.

	* gcc.dg/dfp/bitint-9.c: New test.