[Ada] Fix rounding of fixed-point arithmetic operation

[Ada] Fix rounding of fixed-point arithmetic operation

Fixed-point multiplication, division and conversion may lead to calling
the function Double_Divide in s-arit64 runtime unit. In the special case
where arguments have the special values X = -2**63 and the absolute
value of the product of its other arguments Y*Z = 2**64, the rounded
value should be either -1 or 1, but currently Double_Divide returns a
quotient of 0.

Rounding only applies when Round attribute is called on the arithmetic
operation for a decimal fixed-point result, or the result type is
integer.

This fixes correctly applies rounding away from 0 in that special case.

2019-09-17  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* libgnat/s-arit64.adb (Double_Divide): Correctly handle the
	special case when rounding.

gcc/testsuite/

	* gnat.dg/fixedpnt7.adb: New testcase.

From-SVN: r275796