[Ada] Flag Sec_Stack_Used incorrectly set by ghost code

[Ada] Flag Sec_Stack_Used incorrectly set by ghost code

2019-10-10  Patrick Bernardi  <bernardi@adacore.com>

gcc/ada/

	* bindgen.adb (System_Secondary_Stack_Package_In_Closure):
	Renamed flag System_Secondary_Stack_Used to be clearer of what
	it represents.
	(Gen_Adainit): Refactor secondary stack related code to make it
	clearer.
	* rtsfind.adb (Load_RTU): Don't set Sec_Stack_Used flag here
	(RTE): Set Sec_Stack_Used if the System.Secondary_Stack is
	referenced, but not if we're ignoring ghost code.

From-SVN: r276811