[Ada] Avoid buffer overflow in Long_Long_Float_Text_IO

[Ada] Avoid buffer overflow in Long_Long_Float_Text_IO

2020-06-04  Steve Baird  <baird@adacore.com>

gcc/ada/

	* libgnat/s-imgrea.ads: Declare a named number
	Max_Real_Image_Length with value 5200.  Provide a comment
	explaining why this value was chosen.
	* libgnat/s-imgrea.adb (Set_Image_Real): Increase the upper
	bound of the local String variable Digs to
	Max_Real_Image_Length.
	* libgnat/a-tiflau.adb (Put): Increase the upper bound of the
	local String variable Buf to Max_Real_Image_Length.
	(Puts): Increase the upper bound of the local String variable
	Buf to Max_Real_Image_Length.