rtems-ld does not know aarch64

Summary

rtems-ld does not know aarch64.

Steps to reproduce

rtems-ld -n -e appmain -s -O rap --base rtems.elf build/aarch64-rtems6-rockchip_rk3568/app.elf -o build/aarch64-rtems6-rockchip_rk3568/app.out
error: machine-type: unknown machine type: 183