musl: opt for dynamic linking for arm arch
musl: opt for dynamic linking for arm arch

When building with musl for an arm target link with a shared libc (like mips
already does).

anatol/libc,rust-lang-nursery/libc,philippkeller/libc,anatol/libc,rust-lang-nursery/libc,philippkeller/libc,rust-lang/libc,superriva/libc,rust-lang/libc,anatol/libc,cactorium/libc,cactorium/libc,superriva/libc,superriva/libc,mneumann/libc,zethra/libc,rust-lang-nursery/libc,philippkeller/libc,zethra/libc,rust-lang/libc,mneumann/libc,zethra/libc,cactorium/libc,mneumann/libc