Write ninja file directly to the output file

Writing the ninja file to a byte buffer causes a significant amount
of time to be spent in memmove when growing the byte slice.  Write
the file directly to disk instead.

This also fixes some unhandled error warnings, which become more
likely when doing disk IO instead of byte buffer writes.

Change-Id: I5094e4c45cab4012713037f60c5a4fb00718f92e