Add constructor to bound_minimal_symbol

This adds a constructor to bound_minimal_symbol, to avoid a build
failure with clang that Simon pointed out.

I also took the opportunity to remove some redundant initializations,
and to change one use of push_back to emplace_back, as suggested by
Simon.

gdb/hppa-tdep.c

@@ -2531,7 +2531,7 @@ struct bound_minimal_symbol
 hppa_lookup_stub_minimal_symbol (const char *name,
 				 enum unwind_stub_types stub_type)
 {
-  struct bound_minimal_symbol result = { NULL, NULL };
+  struct bound_minimal_symbol result;
 
   for (objfile *objfile : current_program_space->objfiles ())
     {