um: Use HOST_DIR for mrproper

When HEADER_ARCH was introduced, the MRPROPER_FILES (then MRPROPER_DIRS)
list wasn't adjusted, leaving SUBARCH as part of the path argument.
This resulted in the "mrproper" target not cleaning up arch/x86/... when
SUBARCH was specified. Since HOST_DIR is arch/$(HEADER_ARCH), use it
instead to get the correct path.

Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Azeem Shaikh <azeemshaikh38@gmail.com>
Cc: linux-um@lists.infradead.org
Fixes: 7bbe7204

 ("um: merge Makefile-{i386,x86_64}")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230606222442.never.807-kees@kernel.org