ada: Fix unexpected overflow check before fixed-point multiplication

ada: Fix unexpected overflow check before fixed-point multiplication

The problem is that the code generating the fixed-point multiply uses the
subtypes of the operands to size the operation, while operations are to be
performed in base types, which are signed per the RM 3.5.9(12) subclause.
As a consequence, when the subtypes are fully asymmetric unsigned, the size
is too small and an incorrect overflow check is generated.

The code generating the divide was fixed a long time ago, this aligns the
code generating the multiply and the code generating the remainder, which
in turn requires a couple of adjustments to related routines.

gcc/ada/ChangeLog:

	PR ada/122063
	* exp_fixd.adb (Build_Double_Divide_Code): Convert the result of the
	multiply.
	(Build_Multiply): Use base types of operands to size the operation.
	(Build_Rem): Likewise.
	(Build_Scaled_Divide_Code): Convert the result of the multiply.