libgcc: Add {unsigned ,}__int128 <-> _Decimal{32,64,128} conversion support [PR65833]

libgcc: Add {unsigned ,}__int128 <-> _Decimal{32,64,128} conversion support [PR65833]

The following patch adds the missing
{unsigned ,}__int128 <-> _Decimal{32,64,128}
conversion support into libgcc.a on top of the _BitInt support
(doing it without that would be larger amount of code and I hope all
the targets which support __int128 will eventually support _BitInt,
after all it is a required part of C23) and because it is in libgcc.a
only, it doesn't hurt that much if it is added for some architectures
only in GCC 15.
Initially I thought about doing this on the compiler side, but doing
it on the library side seems to be easier and more -Os friendly.
The tests currently require bitint effective target, that can be
removed when all the int128 targets support bitint.

2023-11-09  Jakub Jelinek  <jakub@redhat.com>

	PR libgcc/65833
libgcc/
	* config/t-softfp (softfp_bid_list): Add
	{U,}TItype <-> _Decimal{32,64,128} conversions.
	* soft-fp/floattisd.c: New file.
	* soft-fp/floattidd.c: New file.
	* soft-fp/floattitd.c: New file.
	* soft-fp/floatuntisd.c: New file.
	* soft-fp/floatuntidd.c: New file.
	* soft-fp/floatuntitd.c: New file.
	* soft-fp/fixsdti.c: New file.
	* soft-fp/fixddti.c: New file.
	* soft-fp/fixtdti.c: New file.
	* soft-fp/fixunssdti.c: New file.
	* soft-fp/fixunsddti.c: New file.
	* soft-fp/fixunstdti.c: New file.
gcc/testsuite/
	* gcc.dg/dfp/int128-1.c: New test.
	* gcc.dg/dfp/int128-2.c: New test.
	* gcc.dg/dfp/int128-3.c: New test.
	* gcc.dg/dfp/int128-4.c: New test.