samples/bpf: Enable cross compiler support

When cross compiling, bpf samples use HOSTCC for compiling the non-BPF part of
the sample, however what we really want is to use the cross compiler to build
for the cross target since that is what will load and run the BPF sample.
Detect this and compile samples correctly.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Joel Fernandes <joelaf@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>