This overcomes the lack of fused multiply-add instruction on the x87
FPU by doing an iterated addition with exact error handling for the
last digit taken into account for the mantissa.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/s-valrea.adb (Fast2Sum): New function.
	(Integer_to_Real): Use it in an iterated addition with exact
	error handling for the case where an extra digit is needed.
	Move local variable now only used in the exponentiation case.