um: ensure `make ARCH=um mrproper` removes arch/$(SUBARCH)/include/generated/

In this workflow:

$ make ARCH=um defconfig && make ARCH=um -j8
  [snip]
$ make ARCH=um mrproper
  [snip]
$ make ARCH=um defconfig O=./build_um && make ARCH=um -j8 O=./build_um
  [snip]
  CC      scripts/mod/empty.o
In file included from ../include/linux/types.h:6,
                 from ../include/linux/mod_devicetable.h:12,
                 from ../scripts/mod/devicetable-offsets.c:3:
../include/uapi/linux/types.h:5:10: fatal error: asm/types.h: No such file or directory
    5 | #include <asm/types.h>
      |          ^~~~~~~~~~~~~
compilation terminated.
make[2]: *** [../scripts/Makefile.build:100: scripts/mod/devicetable-offsets.s] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [/home/iha/sdb/opensource/lkmp/linux-kselftest.git/Makefile:1140: prepare0] Error 2
make[1]: Leaving directory '/home/iha/sdb/opensource/lkmp/linux-kselftest.git/build_um'
make: *** [Makefile:180: sub-make] Error 2

The cause of the error was because arch/$(SUBARCH)/include/generated files
weren't properly cleaned by `make ARCH=um mrproper`.

Fixes: a788b2ed ("kbuild: check arch/$(SRCARCH)/include/generated before out-of-tree build")
Reported-by: Theodore Ts'o <tytso@mit.edu>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Link: https://groups.google.com/forum/#!msg/kunit-dev/QmA27YEgEgI/hvS1kiz2CwAJ


Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>