crt: Add double underscore prefix to the mingw_app_type symbol

This symbol vaguely seems like it might be meant to be overridden
by the app code, but I don't see how that practically would work,
as the init routines (crtdll.c and crtexe.c) set it anyway, so
whatever default value the user code provided wouldn't have any
effect anyway.

Signed-off-by: Martin Storsjö <martin@martin.st>