Using the integrated assembler we'd fail to change section to the
.tbss section for zerofill thread locals. Make sure we do this
before emitting the zerofills.

Fixes PR15972.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187913 91177308-0d34-0410-b5e6-96231b3b80d8