depends: make osx output deterministic

ld64 is threaded, and uses a worker for each CPU to parse input files. But
there's a bug in the parser causing dependencies to be calculated differently
based on which files have already been parsed.

As a result, builders with more CPUs are more likely to see non-determinism.

This looks to have been fixed in a newer version of ld64, so just disable
threading for now. There's no noticible slowdown.

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