ada: Small cleanup and refactoring in expansion of asynchronous select

The exception handler that catches Abort_Signal does nothing nowadays.
This refactors the code to use Build_Abort_Block more consistently and
also makes it simpler by dropping the identifier on the abort block.

No functional changes.

gcc/ada/ChangeLog:

	* exp_sel.ads (Build_Abort_Block): Remove second parameter and
	rename the third.
	(Build_Abort_Block_Handler): Fix description.
	* exp_sel.adb (Build_Abort_Block): Remove second parameter, rename
	the third and adjust accordingly.
	* exp_ch9.adb (Expand_N_Asynchronous_Select): Fix the description
	of the exception handler throughout.  Remove Abort_Block_Ent and
	Hdle local variables.  Call Build_Abort_Block consistently to build
	the abort block and adjust existing calls.