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 <[email protected]>
Cc: Anton Ivanov <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: Azeem Shaikh <[email protected]>
Cc: [email protected]
Fixes: 7bbe720 ("um: merge Makefile-{i386,x86_64}")
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: kees

arch/um/Makefile

@@ -149,7 +149,7 @@ export CFLAGS_vmlinux := $(LINK-y) $(LINK_WRAPS) $(LD_FLAGS_CMDLINE) $(CC_FLAGS_
 # When cleaning we don't include .config, so we don't include
 # TT or skas makefiles and don't clean skas_ptregs.h.
 CLEAN_FILES += linux x.i gmon.out
-MRPROPER_FILES += arch/$(SUBARCH)/include/generated
+MRPROPER_FILES += $(HOST_DIR)/include/generated
 
 archclean:
 	@find . \( -name '*.bb' -o -name '*.bbg' -o -name '*.da' \