libbpf-tools: add support for cross compilation
libbpf-tools: add support for cross compilation

In cross compiling case, we need target toolchain for application code,
and host toolchain for bpftool.

Signed-off-by: Jie Wang <90b7a97e96a46ce96419408498b328e9b4a97ae7@lixiang.com>

iovisor/bcc,iovisor/bcc,iovisor/bcc,iovisor/bcc,iovisor/bcc