MC: Change MCAssembler::writeSectionData and writeFragmentPadding to take a raw_ostream. NFCI.

Also clean up a couple of hacks where we were writing the section
contents to another stream by setting the object writer's stream,
writing and setting it back.

Part of PR37466.

Differential Revision: https://reviews.llvm.org/D47038

llvm-svn: 332858