Since FreeBSD 4.1, the kernel expects binaries to be marked with ELFOSABI_FREEBSD in the ELF header to exec() them.
LLD unconditionally sets OSABI to ELF_OSABINONE, and everything linked with it won't run on FreeBSD (unless explicitly rebranded).
ELF binary type "0" not known.
zsh: exec format error: ./aarch64-hello
FreeBSD could be modified to accept ELF_OSABINONE, but that would break all existing binaries, so the kernel needs to support both ABINONE and ABIFREEBSD. I plan to push this change in FreeBSD one day, which, unfortunately, is not today.
This code patches lld so it sets the header field correctly.
For completeness, the rationale of this change is explained in the FreeBSD commit message, and it's apparently to pleasee binutils maintainers at the time.