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: a788b2e ("kbuild: check arch/$(SRCARCH)/include/generated before out-of-tree build")
Reported-by: Theodore Ts'o <[email protected]>
Suggested-by: Masahiro Yamada <[email protected]>
Signed-off-by: Vitor Massaru Iha <[email protected]>
Reviewed-by: Brendan Higgins <[email protected]>
Tested-by: Brendan Higgins <[email protected]>
Link: https://groups.google.com/forum/#!msg/kunit-dev/QmA27YEgEgI/hvS1kiz2CwAJ
Signed-off-by: Masahiro Yamada <[email protected]>

Author: iha

arch/um/Makefile

@@ -140,6 +140,7 @@ export CFLAGS_vmlinux := $(LINK-y) $(LINK_WRAPS) $(LD_FLAGS_CMDLINE)
 # When cleaning we don't include .config, so we don't include
 # TT or skas makefiles and don't clean skas_ptregs.h.
 CLEAN_FILES += linux x.i gmon.out
+MRPROPER_DIRS += arch/$(SUBARCH)/include/generated
 
 archclean:
 	@find . \( -name '*.bb' -o -name '*.bbg' -o -name '*.da' \