Building hafnium on arm64 hosts isn't possible out of the box:
```
./prebuilts/linux-x64/gn/gn: 1: ELF: not found
./prebuilts/linux-x64/gn/gn: 2: @: not found
./prebuilts/linux-x64/gn/gn: 1: Syntax error: end of file unexpected (expecting ")"
```
The Makefile sets `PREBUILTS` to `.../linux-x64`, so tries to run x86-64 binaries on an arm64 host.
If I edit the Makefile to use `linux-aarch64`, then the build fails as there is no `gn` binary.
After setting `GN` and `NINJA` to external binaries, the build still fails as x86 is hard-coded in `build/image/dtc.py`:
```
DTC_ROOT = os.path.join(HF_ROOT, "prebuilts", "linux-x64", "dtc")
```