Add a InitSections method to the streamer interface.

Add a InitSections method to the streamer interface.

The ELF implementation now creates text, data and bss to match the gnu as
behavior.

The text streamer still has the old MachO specific behavior since
the testsuite checks that it will error when a directive is given
before a setting the current section for example.

A nice benefit is that -n is not required anymore when producing
ELF files.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114027 91177308-0d34-0410-b5e6-96231b3b80d8