gdb/testsuite: fix running gdb.python/py-explore-cc with clang

The test gdb.python/py-explore-cc.exp was showing one unexpected
failure. This was due to how clang mapped instructions to lines,
resulting in the inferior seemingly stopping at a different location.

This patch adds a nop line in the relevant location so we don't need to
add XFAILs for existing clang releases, if this gets solved in future
versions.

Approved-By: Tom Tromey <tom@tromey.com>
diff --git a/gdb/testsuite/gdb.python/py-explore.cc b/gdb/testsuite/gdb.python/py-explore.cc
index dfbc08c..e526c3c 100644
--- a/gdb/testsuite/gdb.python/py-explore.cc
+++ b/gdb/testsuite/gdb.python/py-explore.cc
@@ -42,7 +42,8 @@
   b.i = 10;
   b.c = 'a';
 
-  return 0; /* Break here.  */
+  val *= 1; /* Break here.  */
+  return 0;
 }
 
 int