Update and Fix rust linking

• Update rust to our latest toolchain
  • Requires rust!24 (merged)
• Replace MAGIC_EXTRA_RUSTFLAGS with LDFLAGS_x86_64_unknown_redox and CARGO_TARGET_X86_64_UNKNOWN_REDOX_RUSTFLAGS
  • This is a clearer (but undocumented?) way to set LDFLAGS specifically for stage2 compiler
  • Adapt CARGO_TARGET_X86_64_UNKNOWN_REDOX_RUSTFLAGS to pass a similar LDFLAGS
• Unset RUSTFLAGS as it's redundant (equivalent to crt-static = false for stage2) and poisoning the stage1 compiler
• Move rust binaries to /usr to fix issues with auto deps
• Remove gcc13 dependency. GCC is only used in Rust as a backend alternative of LLVM and only works for Linux x64
• Remove strip process. It's already handled automatically by cookbook.
• Adapt config.toml explanation:
  • Reduced LLVM targets for faster compilation time
  • Move rpath = false to specifically only use it for stage2 compiler
    • This is because it's poisoning the stage1 compiler, it can't find the necessary shared lib when it running.