gdb: make Python conftest compatible with Python limited C API

The current test to check the support of '--dynamic-list' linker flag
uses PyRun_SimpleString (), which is part of the unstable API. As it is
now, the test will systematically fail due to the undefined symbol
rather than testing the import of ctypes.
This patch replaces PyRun_SimpleString () by an equivalent code relying
on the limited C API, and compatible with Python 3.4.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23830
Approved-By: Tom Tromey <tom@tromey.com>