windows pydevices variant: run built-in module __init__ on import

[bdbarnett]

For PyDevices/displayif#41.

With this, micropython.exe registers jpegio's LVGL decoder on import jpegio the way unix does. The windows port is at the core ROM level, where MICROPY_MODULE_BUILTIN_INIT is off, so jpegio's __init__ never ran there. The pydevices windows variant now turns it on. That's one line in variants/windows/pydevices/mpconfigvariant.h, and the vst3-engine variant is unchanged.

I built it with upstream's make, cross-compiled, into its own build directory (ports/windows/build-desk-jpegio, the kitchen-sink manifest, under the build lock), and ran the same script on each interpreter:

import lvgl as lv; lv.init(); import jpegio; print(jpegio.lvgl_decoders())

this build            win32  ('jpegio', 'LODEPNG', 'BIN')
bin/micropython.exe   win32  ('LODEPNG', 'BIN')
bin/micropython       linux  ('jpegio', 'LODEPNG', 'BIN')

displayif's tests/jpegio/test_jpegio_lvgl.py passes in full on this build: 3 exact digests, the file source, memory, and the deinit/init cycle. PyDevices/displayif#43 makes that test require import-time registration on win32. It fails on the current bin/micropython.exe, so it catches the missing hook.

After this merges, tools/build_interpreters.sh --only mp-windows in the anchor puts it in bin/micropython.exe. I didn't rebuild the shared binaries.