[yaml2obj][ELF] Align section contents in the output.

The improperly aligned section content in the output was causing
buildbot failures. This should fix them.

Incidentally, this results in a simpler and more robust API for
ContiguousBlobAccumulator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184621 91177308-0d34-0410-b5e6-96231b3b80d8