[gdb/testsuite] Fix gdb.gdb/python-helper.exp with -O2 -flto=auto and gcc 7.5.0

[gdb/testsuite] Fix gdb.gdb/python-helper.exp with -O2 -flto=auto and gcc 7.5.0

With a gdb build with -O2 -flto=auto using gcc 7.5.0, I run into:
...
(gdb) ptype global_c^M
^M
Thread 1 "xgdb" hit Breakpoint 3, \
  _Z12c_print_typeP4typePKcP7ui_fileii8languagePK18type_print_options () at \
  gdb/c-typeprint.c:175^M
175     {^M
(outer-gdb) FAIL: gdb.gdb/python-helper.exp: hit breakpoint in outer gdb again
...

This is a problem with the debug info, which marks the CU containing the
function declaration as C rather than C++.  This is fixed in gcc 8 and later.

Work around this compiler problem by allowing the mangled name.

Tested on x86_64-linux.

PR testsuite/30648
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30648