Make GDB compile with Python 3 on MinGW

Make GDB compile with Python 3 on MinGW

PyFile_FromString and PyFile_AsFile have been removed in Python 3.
There is no obvious replacement that works here, and we can't just
pass our FILE* to a DLL in Windows because it may use a different
C runtime.

So we just call a Python function which reads and executes file
contents. Care must be taken to execute it in the context of
__main__.

Tested by inverting the ifdef and running the testsuite on Debian
Linux (even without the patch, I failed at running the testsuite
on Windows). I did test with both Python 2 and 3.

gdb/ChangeLog:

2019-08-22  Christian Biesinger  <cbiesinger@google.com>

	* python/lib/gdb/__init__.py (_execute_file): New function.
	* python/python.c (python_run_simple_file): Call gdb._execute_file
	on Windows.