[gdb/testsuite] Fix xpass in gdb.python/lib-types.exp

When running gdb.python/lib-types.exp, we have an xpass:
...
(gdb) python print (str (typedef_const_typedef_class1_ref_obj.type))^M
typedef_const_typedef_class1_ref^M
(gdb) XPASS: gdb.python/lib-types.exp: \
  python print (str (typedef_const_typedef_class1_ref_obj.type)) \
  (PRMS gcc/55641)
...

When running the same with gcc 4.8, we have an xfail instead:
...
(gdb) python print (str (typedef_const_typedef_class1_ref_obj.type))^M
const typedef_const_typedef_class1_ref^M
(gdb) XFAIL: gdb.python/lib-types.exp: \
  python print (str (typedef_const_typedef_class1_ref_obj.type)) \
  (PRMS gcc/55641)
...

Fix the xpass by xfailing only for the gcc 4.8 pattern.

Tested on x86_64-linux, with:
- gcc 7.5.0
- gcc 4.8.5
- clang 5.0.2

gdb/testsuite/ChangeLog:

2020-02-19  Tom de Vries  <tdevries@suse.de>

	* gdb.python/lib-types.exp: Make xfail more strict.