Fix PhutilLibraryMapBuilder to call the right function in log()

Summary:
arc liberate has been broken for a while because the log function attempts to call fwrite (whose third argument is an integer length) instead of fprintf. I think maybe PHP 5 was more lenient about this? Anyway, this bug makes other development somewhat tricky.

The arcanist source in general seems to be split between fwrite(STDERR, $message."\n") and fprintf(STDERR, "%\n", $message); I decided to go with the latter.

This is sort of a test balloon on submitting diffs to Phorge. Please feel free to let me know if there's anything you'd like done differently!

Test Plan: Ran arc liberate to regenerate source maps and didn't get any errors

Reviewers: O1 Blessed Committers, eax

Reviewed By: O1 Blessed Committers, eax

Subscribers: chris, speck, tobiaswiese

Differential Revision: https://we.phorge.it/D25017