Check whether gnatmake can link with -shared

Currently, gnat-llvm does not ship a shared libgnat.  This patch
changes the relevant test to check whether linking with -shared
actually works.

gdb/testsuite/gdb.ada/win_fu_syms.exp

@@ -16,6 +16,7 @@
 load_lib "ada.exp"
 
 require allow_ada_tests
+require ada_shared_link
 
 standard_ada_testfile foo
 

gdb/testsuite/lib/ada.exp

@@ -289,3 +289,12 @@ gdb_caching_proc ada_minimal_encodings {} {
 gdb_caching_proc ada_og {} {
     return [ada_simple_compile gnat_og additional_flags=-Og]
 }
+
+# Return 1 if GNAT can link with -shared.
+gdb_caching_proc ada_shared_link {} {
+    return [ada_simple_compile ada_shared_link {
+	additional_flags=-bargs
+	additional_flags=-shared
+	additional_flags=-margs
+    }]
+}