llvm latest svn commit is broken as of 2019/01/12. This patch pins the llvm+lld versions used for arm compiles.
It also tweaks the architecture detection to be more reliable in multi-lib/multi-arch situations where the CPU is 64bit but the runtime is 32bit.
* Update plume-front build to use a shell script that adds the necessary dependencies on arm (x86 left untouched inside the script)
* Fix cleanup portion of plume-front.sh
* Break wasm dependencies for arm out so they are run before the main builds to improve layer caching on subsequent docker builds
* Fixup dockerfile so wasm-deps can run properly and with fewer assumptions
* Move new scripts to script/ folder
* Remove saving of path as it's not actually not necessary
* Add rust-toolchain to early copy so it pulls the right rust release (addresses #400)
