Make wxFile::ReadAll() work for unseekable files too

Calling this function with an unseekable file, such as any file under
/sys on Linux systems, would previously just hang as the loop condition
was never satisfied when length was -1.

Fix this by checking for this case and using a different approach by
extending the buffer we read the data into as we go instead of
preallocating it all at once.

See #9965.

docs/changes.txt

@@ -106,6 +106,7 @@ All:
 - Add UTF-8 support to wxZipOutputStream (Tobias Taschner).
 - Update all bundled 3rd party libraries to their latest versions.
 - Use unique prefix for all zlib symbols to avoid link conflicts.
+- Make wxFile::ReadAll() work for unseekable files too.
 
 All (GUI):