make samples/bpf/ using gcc instead of clang/llvm

[Joel Fernandes]

Hi Brendan, Alexei,

I noticed your patch fixing the $subject issue.

https://patchwork.ozlabs.org/patch/471118/

However, I still see make samples/bpf/ using gcc instead of clang.

Here's a verbose kbuild output of make samples/bpf/. My kernel version
is 4.4. It looks to me like Kbuild is ignoring the Makefile in
samples/bpf/. Let me know if you spot anything of the ordinary?
Thanks.

 make  KBUILD_VERBOSE=1 samples/bpf/
[..]
make KBUILD_MODULES=1 \
    -f /home/joel/data/repo/linux-4.4/scripts/Makefile.build obj=samples/bpf
(cat /dev/null; ) > samples/bpf/modules.order
  gcc -Wp,-MD,samples/bpf/.test_verifier.o.d -Isamples/bpf -Wall
-Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer
-std=gnu89  -I/home/joel/data/repo/linux-4.4/./usr/include
-I./usr/include -c -o samples/bpf/test_verifier.o
/home/joel/data/repo/linux-4.4/samples/bpf/test_verifier.c
/home/joel/data/repo/linux-4.4/samples/bpf/test_verifier.c:12:23:
fatal error: linux/bpf.h: No such file or directory
 #include <linux/bpf.h>
                       ^
compilation terminated.
make[2]: *** [samples/bpf/test_verifier.o] Error 1