Add support for `aarch64-unknown-linux-pauthtest`

This PR adds support for aarch64-unknown-linux-pauthtest, a target that enables Pointer Authentication Code (PAC) support in Rust on AArch64 ELF based Linux systems. It uses the aarch64-unknown-linux-pauthtest LLVM target and a pointer-authentication-enabled sysroot with a custom [musl](https://github.com/access-softek/musl) as a reference libc implementation. Dynamic linking is required, with a dynamic linker acting as the ELF interpreter that can resolve pauth relocations and enforce pointer authentication constraints. Please consult a rust-lang PR for the details on the target: https://github.com/rust-lang/rust/pull/155722