builtins: Fix up DFP ICEs on __builtin_fpclassify [PR102674]

builtins: Fix up DFP ICEs on __builtin_fpclassify [PR102674]

This patch is similar to the one I've just posted, __builtin_fpclassify also
needs to print decimal float minimum differently and use real_from_string3.
Plus I've done some formatting fixes.

2024-11-26  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/102674
	* builtins.cc (fold_builtin_fpclassify): Use real_from_string3 rather
	than real_from_string.  Use "1E%d" format string rather than "0x1p%d"
	for decimal float minimum.  Formatting fixes.

	* gcc.dg/dfp/pr102674.c: New test.

(cherry picked from commit 5bb36d832c955e575bd458a02f3c6c5b28564aed)