don't link libc

[Twey]

It is not necessary to link libc manually, as libc is already
contained in liblibc.rlib which gets linked in automatically.

This link annotation is harmful as in static situations (where
target_feature="crt-static") it will force linking the dynamic libc,
producing chimera binaries or breaking the build entirely.

[yuqitao]

I built my project with cargo build --release --target aarch64-unknown-linux-musl.
I got this rust-lang/rust#46651 first. So I add rustflags = [ "-C", "target-feature=+crt-static", "-C", "link-arg=-lgcc" ]. then build successfully. But when I run it, I got Segmentation fault. It seems that we can solve this problem with your commit.