yaml2obj: Support bigobj

Teach yaml2obj how to make a bigobj COFF file.  Like the rest of LLVM,
we automatically decide whether or not to use regular COFF or bigobj
COFF on the fly depending on how many sections the resulting object
would have.

This ends the task of adding bigobj support to LLVM.

N.B. This was tested by forcing yaml2obj to be used in bigobj mode
regardless of the number of sections.  While a dedicated test was
written, the smallest I could make it was 36 MB (!) of yaml and it still
took a significant amount of time to execute on a powerful machine.