Fix comparison of decimal float zeroes (PR80692)

Fix comparison of decimal float zeroes (PR80692)

Decimal float negative zero should compare equal to positive zero.
Decimal float zeroes are encoded as value class "normal" (in real.c);
they need to be handled specially, but in this one case that does not
yet happen.  This fixes it.

	PR middle-end/80692
	* real.c (do_compare): Give decimal_do_compare preference over
	comparing just the signs.

gcc/testsuite/
	PR middle-end/80692
	* gcc.c-torture/execute/pr80692.c: New testcase.

From-SVN: r248174