Simplify local toolchain development

Testing updates to https://github.com/cloudera/native-toolchain currently takes some work. https://cwiki.apache.org/confluence/display/IMPALA/Building+native-toolchain+from+scratch+and+using+with+Impala mentions it's for advanced users only, and the instructions don't quite work anymore. They also don't work very well when switching between different branches.

For aarch64 builds we make this somewhat simpler. Expand on that to make it easier to work with local native-toolchain checkouts.