Hook up "--threads" to gn help.
Hook up "--threads" to gn help.

This was documented but the flag was not hooked up to the help system so it didn't actually appear in "gn help switches".

Review URL: https://codereview.chromium.org/1357523003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#349500}

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