um: let 'make clean' properly clean underlying SUBARCH as well

Building the kernel with O= is affected by stale in-tree build artifacts.

So, if the source tree is not clean, Kbuild displays the following:

  $ make ARCH=um O=build defconfig
  make[1]: Entering directory '/.../linux/build'
  ***
  *** The source tree is not clean, please run 'make ARCH=um mrproper'
  *** in /.../linux
  ***
  make[2]: *** [/.../linux/Makefile:673: outputmakefile] Error 1
  make[1]: *** [/.../linux/Makefile:248: __sub-make] Error 2
  make[1]: Leaving directory '/.../linux/build'
  make: *** [Makefile:248: __sub-make] Error 2

Usually, running 'make mrproper' is sufficient for cleaning the source
tree for out-of-tree builds.

However, building UML generates build artifacts not only in arch/um/,
but also in the SUBARCH directory (i.e., arch/x86/). If in-tree stale
files remain under arch/x86/, Kbuild will reuse them instead of creating
new ones under the specified build directory.

This commit makes 'make ARCH=um clean' recurse into the SUBARCH directory.

Reported-by: Shuah Khan <skhan@linuxfoundation.org>
Closes: https://lore.kernel.org/lkml/20250502172459.14175-1-skhan@linuxfoundation.org/


Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>