[Ada] Disable lock free protected implementation if target lacks support

[Marc Poulhiès via Gcc-patches]

Do not select a lock free implementation if Targparm.Support_Atomic_Primitives
is False (which indicates that the target cannot support it).

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

gcc/ada/

        * sem_ch9.adb
        (Allows_Lock_Free_Implementation): Return False if
        Support_Atomic_Primitives is False.
diff --git a/gcc/ada/sem_ch9.adb b/gcc/ada/sem_ch9.adb
--- a/gcc/ada/sem_ch9.adb
+++ b/gcc/ada/sem_ch9.adb
@@ -64,6 +64,7 @@ with Sinfo;          use Sinfo;
 with Sinfo.Nodes;    use Sinfo.Nodes;
 with Sinfo.Utils;    use Sinfo.Utils;
 with Style;
+with Targparm;       use Targparm;
 with Tbuild;         use Tbuild;
 with Uintp;          use Uintp;
 
@@ -649,6 +650,10 @@ package body Sem_Ch9 is
             --  Start of processing for Satisfies_Lock_Free_Requirements
 
             begin
+               if not Support_Atomic_Primitives_On_Target then
+                  return False;
+               end if;
+
                --  Get the number of errors detected by the compiler so far
 
                if Lock_Free_Given then