build.rs: emit position-independent code when compiling on ARM64

[belowm]

Currently, this crate cannot be used in ARM64 projects: While the crate itself compiles just fine, a binary project depending on libxdp_sys fails to compile:

/usr/bin/ld: /home/.../target/debug/deps/liblibxdp_sys-7ecba993bf090f40.rlib(libxdp.o): relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `stderr@@GLIBC_2.17' which may bind externally can not be used when making a shared object; recompile with -fPIC 

This PR adds the CFLAGS -fPIC -pie when invoking make for libxdp, which fixes the issue (well, at least for me).

[chenhengqi]

@KarstenB Please help reviewing this PR. Thanks.

[KarstenB]

What exactly are you trying to do? I use liblibxdp_sys via xsk-rs on my Mac in ARM64 linux without any issues. Are you trying to create a statically linked binary with musl?
A simple reproducable .devcontainer example would be awesome.