samples/bpf: Use target CC environment for HDR_PROBE

No need in hacking HOSTCC to be cross-compiler any more, so drop
this trick and use target CC for HDR_PROBE.

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/20191011002808.28206-11-ivan.khoronzhuk@linaro.org