Crashlytics: Add dispatch_once for opening sdk log file

[cltnschlosser]

Resolves #5903

I had a more complicated change earlier, but settled on this. The file is never closed, so this will work, and should be better than using a queue or semaphore.

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[cltnschlosser]

@googlebot I signed it!

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[morganchen12]

Thanks!

Waiting for @samedson approval before merging, since this code may have non-obvious constraints.

[samedson]

Hey @cltnschlosser - thanks for the fix! This looks LGTM