to compile NetBSD you have to clone the repo first on some place by your choice
https://github.com/NetBSD/src.git
then cd into it using
cd src
and then you have to build the toolchain using the build.sh script the next way:
./build.sh -U -O ~/obj -j8 -m evbarm -a aarch64 tools
you'll have to replace the -j8 depending on how many cores you have on your processor, if you don't know how many you have, you can check with
nproc --all
tried on two arch machines, didn't work, tried on a debian machine and it worked out perfectly.