[Ada] Secondary stack and a-tags

[Ada] Secondary stack and a-tags

The simple use of Ada.Tags triggers a dependency on the secondary stack
mechanism, which is unwanted on small embedded targets. To avoid this
dependency, we special case a-tags.ali in ALI.Scan_ALI to not set
Sec_Stack_Used. If some other code calls one of the functions returning
a string, this code will also be marked as requiring the secondary
stack. We also remove the need to import and set __gnat_binder_ss_count
in this case by ensuring this variable defaults to 0.

gcc/ada/

	* ali.adb (Scan_ALI): Special case a-tags.ali when setting
	Sec_Stack_Used.
	* bindgen.adb (Gen_Adainit): Simplify handling of secondary
	stack related code, and only import __gnat_binder_ss_count when
	needed.
	* libgnat/s-secsta.adb (Binder_SS_Count): Default initialize to
	0.