[Ada] Improve performance of conversion from String to Long_Float

[Ada] Improve performance of conversion from String to Long_Float

Once it is sure that the result will be infinity, stop computation and return
the result. This ensure that the function call duration is bounded. Before that
change on some cases the computation was taking more than a few seconds.

2018-05-25  Nicolas Roche  <roche@adacore.com>

gcc/ada/

	* libgnat/s-valrea.adb (Scan_Real): Abort computation once it is sure
	that the result will be either -infinite or +infinite.

From-SVN: r260743