[Ada] Elaboration order v4.0 and linker switches

[Ada] Elaboration order v4.0 and linker switches

This patch adds a missing functionality with respect to elaboration
order v3.0.  Units carry an attribute called Elab_Position which among
other things controls the sorting of linker switches by gnatbind.
Setting the proper position ensures the gnatbind will output the linker
switches in an order compatible with what gnatlink expects.

No simple test available as this requires a Windows cross-compiler.

2019-07-10  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* bindo-elaborators.adb (Elaborate_Units): Set attribute
	Elab_Position of all elaborated units.
	(Set_Unit_Elaboration_Positions): New routine.

From-SVN: r273326