kbuild: Fix GNU make v3.80 compatibility

According to Documentation/Changes, the kernel should be buildable with
GNU make 3.80+.  Commit 88d7be03 (kbuild:
Use a single clean rule for kernel and external modules) introduced the
"$(or" construct, which requires make 3.81.  This causes "make clean" to
malfunction when it is used with external modules.

Replace "$(or" with an equivalent "$(if" expression, to restore backward
compatibility.
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -1398,7 +1398,7 @@ endif # KBUILD_EXTMOD
 clean: $(clean-dirs)
 	$(call cmd,rmdirs)
 	$(call cmd,rmfiles)
-	@find $(or $(KBUILD_EXTMOD), .) $(RCS_FIND_IGNORE) \
+	@find $(if $(KBUILD_EXTMOD), $(KBUILD_EXTMOD), .) $(RCS_FIND_IGNORE) \
 		\( -name '*.[oas]' -o -name '*.ko' -o -name '.*.cmd' \
 		-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \
 		-o -name '*.symtypes' -o -name 'modules.order' \