samples/bpf: Use own EXTRA_CFLAGS for clang commands

It can overlap with CFLAGS used for libraries built with gcc if
not now then in next patches. Correct it here for simplicity.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191011002808.28206-5-ivan.khoronzhuk@linaro.org