[SM78 1/2] Upgrade Spidermonkey build system and binaries to 78.6.0
SM78 drops usage of Python 2 in favour of Python 3.
SM78 is the first ESR to actually use rust, which means a new static library is bundled.
This means LLVM-objdump is now a dependency of Spidermonkey, and also 0 A.D.
SM78 also removes the need for NSPR on Windows (Bug 1556646).
Tested by: Stan, Freagarach, mammadori
Differential Revision: https://code.wildfiregames.com/D3167