[Ada] Introduce -gnatd_A to set Opt.Disable_ALI_File

[Ada] Introduce -gnatd_A to set Opt.Disable_ALI_File

This will allow us to remove the import of flag_compare_debug in
lib-writ.adb in a second stage.

2018-09-26  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* gnat1drv.adb (Adjust_Global_Switches): -gnatd_A sets
	Opt.Disable_ALI_File.
	* debug.adb: Update debug flags documentation.

From-SVN: r264605