Makeconfig: Fix time64-compat.mk target

The time64-compat.i needed for the time64-compat.mk Makeconfig include
ends up trying to include libc-modules.h, which is not generated until
later.  This results in an error during build which is not noticed
because it somehow does not terminate the build.

Exclude the modules cppflags from the pre-processing command and fix
up the time64-compat.mk target so that the build runs without issues.

Reviewed-by: Andreas Schwab <schwab@linux-m68k.org>