Fix gdb.base/list-ambiguous.exp with Clang

The ambiguous variable parts of gdb.base/list-ambiguous.exp failed
when built with Clang because the variable in question was unused
and was optimized out.  This commit adds __attribute__ ((used)) to
both definitions.

gdb/testsuite/ChangeLog:

	* gdb.base/list-ambiguous0.c (ambiguous_var): Add
	__attribute__ ((used)).
	* gdb.base/list-ambiguous1.c (ambiguous_var): Likewise.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-10-06  Gary Benson <gbenson@redhat.com>
+
+	* gdb.base/list-ambiguous0.c (ambiguous_var): Add
+	__attribute__ ((used)).
+	* gdb.base/list-ambiguous1.c (ambiguous_var): Likewise.
+
 2020-10-06  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.base/m32r.ld: Remove SEARCH_DIR line.  Add MEMORY regions,

gdb/testsuite/gdb.base/list-ambiguous0.c

@@ -25,7 +25,7 @@
 
 static void __attribute__ ((used)) ambiguous_fun (void) {}
 
-static int ambiguous_var;
+static int __attribute__ ((used)) ambiguous_var;
 
 
 

gdb/testsuite/gdb.base/list-ambiguous1.c

@@ -28,7 +28,7 @@
    "list"'s behavior with ambiguous linespecs.  */
 static void __attribute__ ((used)) ambiguous_fun (void) {}
 
-static int ambiguous_var;
+static int __attribute__ ((used)) ambiguous_var;