[gdb/testsuite] Add xfail in gdb.cp/subtypes.exp

[gdb/testsuite] Add xfail in gdb.cp/subtypes.exp

When running test-case gdb.cp/subtypes.exp with gcc 4.8.4, we run into:
...
FAIL: gdb.cp/subtypes.exp: ptype main::Foo
FAIL: gdb.cp/subtypes.exp: ptype main::Bar
FAIL: gdb.cp/subtypes.exp: ptype main::Baz
FAIL: gdb.cp/subtypes.exp: ptype foobar<int>::Foo
FAIL: gdb.cp/subtypes.exp: ptype foobar<int>::Bar
FAIL: gdb.cp/subtypes.exp: ptype foobar<int>::Baz
FAIL: gdb.cp/subtypes.exp: ptype foobar<char>::Foo
FAIL: gdb.cp/subtypes.exp: ptype foobar<char>::Bar
FAIL: gdb.cp/subtypes.exp: ptype foobar<char>::Baz
...

The problem is gcc PR debug/55541, which generates a superfluous
DW_TAG_lexical_block.

Add a corresponding xfail.

Tested on x86_64-linux.