[MC] Remove MCAssembler's copy of OS

We can just ask the ObjectWriter for it's stream instead of caching
around our own reference to it.  No functionality change is intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246604 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed