`devkit`[1] is the de facto tool to establish a complete build environment for cross-compilation. `open/doc/building.md` states that

> `BASE_OS` must be one of "OEL6" for Oracle Enterprise Linux 6 or "Fedora" (if
> not specified "OEL6" will be the default).

Though `OEL6` is an invalid `BASE_OS`, it must be `OL` instead:

```
$ make TARGETS=aarch64-linux-gnu BASE_OS=OEL6
...
Tools.gmk:90: *** Unknown base OS OEL6. Stop.
```

[~mikael] commented that

> the BASE_OS no longer includes the version.
> Looks like JDK-8220093 missed updating building.md

[1] https://openjdk.org/groups/build/doc/building.html#using-openjdk-devkits