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>