[Ada] Don't create calls to Abort_Undefer when not Abort_Allowed

[Pierre-Marie de Rodat via Gcc-patches]

Prevent creation of references to Abort_Undefer when aborts aren't
allowed. Another solution could have been an early return at
Expand_N_Asynchronous_Select's beginning, but this would break backends
that currently expect trees that do not contain any
N_Asynchronous_Selects in their AST (e.g. CodePeer).

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * exp_ch9.adb (Expand_N_Asynchronous_Select): Don't generate
        Abort_Undefers when not Abort_Allowed.
diff --git a/gcc/ada/exp_ch9.adb b/gcc/ada/exp_ch9.adb
--- a/gcc/ada/exp_ch9.adb
+++ b/gcc/ada/exp_ch9.adb
@@ -7812,7 +7812,9 @@ package body Exp_Ch9 is
 
          Hdle := New_List (Build_Abort_Block_Handler (Loc));
 
-         Prepend_To (Astats, Build_Runtime_Call (Loc, RE_Abort_Undefer));
+         if Abort_Allowed then
+            Prepend_To (Astats, Build_Runtime_Call (Loc, RE_Abort_Undefer));
+         end if;
 
          Abortable_Block :=
            Make_Block_Statement (Loc,