speedo: Enable additional runtime protections on Windows.

* build-aux/speedo.mk (speedo_w32_cflags): Remove -mms-bitfields
because it is for a long time the gcc default.  Enable control flow
protection.

Note that due to mingw static linking problems with libssp the stack
protector is not yet enabled.

(cherry picked from commit afe87ffc08e14317f4ef5bbe2940d07203a43808)