[gdb/testsuite] Fix gdb.gdb/python-helper.exp with gdb built with -flto

[gdb/testsuite] Fix gdb.gdb/python-helper.exp with gdb built with -flto

With a gdb build with gcc 7.5.0 and "-O2 -flto=auto -g", I run into:
...
(outer-gdb) PASS: gdb.gdb/python-helper.exp: print varobj_table
print inferior_list
$5 = {m_front = 0x212e830, m_back = 0x2e39aa0}
(outer-gdb) FAIL: gdb.gdb/python-helper.exp: print inferior_list
...

The problem is that the type of inferior_list:
...
(outer-gdb) what inferior_list^M
type = intrusive_list^M
(outer-gdb)
...
is not descriptive enough to trigger the pretty pretter.

Note that with a gdb build with -O0, we'd get instead:
...
(outer-gdb) what inferior_list^M
type = intrusive_list<inferior, intrusive_base_node<inferior> >
(outer-gdb)
...

Fix this by detecting this situation, and declaring the test unsupported.

Tested on x86_64-linux.