Auto merge of #14609 - upsuper:auto-regen-atoms, r=Wafflespeanut

Update atoms when build geckolib with gecko

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This would automatically update the generated atom-related files in-tree before calling cargo to build geckolib. With this change, `./mach build-gecko --with-gecko` should be now the easiest way to update all in-tree bindings.

This isn't really a build-time generating, because it isn't invoked from the build script, but from mach. It isn't done the other way because
1. it doesn't seem to be a common practice for build script to write anything to the src directory
2. real build-time generated atom data wouldn't need to include atoms for multiple platforms, but we still want it so that we can build geckolib independently

cc @heycam

r? @emilio

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