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.

gdb/python/lib/gdb/__init__.py

@@ -106,6 +106,32 @@ def _execute_unwinders(pending_frame):
 
     return None
 
+def _execute_file(filepath):
+    """This function is used to replace Python 2's PyRun_SimpleFile.
+
+    Loads and executes the given file.
+
+    We could use the runpy module, but its documentation says:
+    "Furthermore, any functions and classes defined by the executed code are
+    not guaranteed to work correctly after a runpy function has returned."
+    """
+    globals = sys.modules['__main__'].__dict__
+    set_file = False
+    # Set file (if not set) so that the imported file can use it (e.g. to
+    # access file-relative paths). This matches what PyRun_SimpleFile does.
+    if not hasattr(globals, '__file__'):
+        globals['__file__'] = filepath
+        set_file = True
+    try:
+        with open(filepath, 'rb') as file:
+            # We pass globals also as locals to match what Python does
+            # in PyRun_SimpleFile.
+            compiled = compile(file.read(), filepath, 'exec')
+            exec(compiled, globals, globals)
+    finally:
+        if set_file:
+            del globals['__file__']
+
 
 # Convenience variable to GDB's python directory
 PYTHONDIR = os.path.dirname(os.path.dirname(__file__))