Bug 1619460 - ensure subsequent runs of `./mach build-backend` under Python 3...

Bug 1619460 - ensure subsequent runs of `./mach build-backend` under Python 3 produce consistent results r=dmajor

Dictionary iteration under Python 3 is in an inherently unpredictable order, and while we try to keep DEFINES ordered through the use of OrderedDicts, if at any point we populate DEFINES directly or indirectly while iterating through the contents of a non-ordered dictionary, the order of the DEFINES (and therefore the contents of the output Makefile) will be nondeterministic as well. This patch makes a number of changes to ensure that we only ever populate DEFINES in a deterministic fashion. (Note that in Python 3.7 and later, the built-in dict class actually has deterministic ordering, so these changes are technically only necessary until our minimum Python version becomes 3.7.)

Differential Revision: https://phabricator.services.mozilla.com/D66089

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extra : moz-landing-system : lando