add aarch64-unknown-openbsd definition

src/unix/bsd/apple/mod.rs

src/unix/bsd/freebsdlike/mod.rs

src/unix/bsd/mod.rs

src/unix/bsd/netbsdlike/netbsd/mod.rs

src/unix/bsd/netbsdlike/openbsdlike/bitrig/mod.rs

src/unix/bsd/netbsdlike/openbsdlike/bitrig/x86.rs

src/unix/bsd/netbsdlike/openbsdlike/bitrig/x86_64.rs

src/unix/bsd/netbsdlike/openbsdlike/mod.rs

src/unix/bsd/netbsdlike/openbsdlike/openbsd/aarch64.rs

src/unix/bsd/netbsdlike/openbsdlike/openbsd/mod.rs

src/unix/bsd/netbsdlike/openbsdlike/openbsd/x86.rs

src/unix/bsd/netbsdlike/openbsdlike/openbsd/x86_64.rs

src/unix/bsd/netbsdlike/openbsdlike/other/mod.rs