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