ada: Introduce -gnateH switch to force reverse Bit_Order threshold to 64

ada: Introduce -gnateH switch to force reverse Bit_Order threshold to 64

This can be helpful for legacy code that still makes use of an original
reverse Bit_Order clause, i.e. without a Scalar_Storage_Order clause.

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst (Compiler
	Switches): Document -gnateH.
	* opt.ads (Reverse_Bit_Order_Threshold): New variable.
	* sem_ch13.adb (Adjust_Record_For_Reverse_Bit_Order): Use its value
	if it is nonnegative instead of System_Max_Integer_Size.
	* switch-c.adb (Scan_Front_End_Switches): Deal with -gnateH.
	* usage.adb (Usage): Print -gnateH.
	* gnat_ugn.texi: Regenerate.