Adds support for writing the .bss section for XCOFF object files.

Adds support for writing the .bss section for XCOFF object files.

Adds Wrapper classes for MCSymbol and MCSection into the XCOFF target
object writer. Also adds a class to represent the top-level sections, which we
materialize in the ObjectWriter.

executePostLayoutBinding will map all csects into the appropriate
container depending on its storage mapping class, and map all symbols
into their containing csect. Once all symbols have been processed we
- Assign addresses and symbol table indices.
- Calaculte section sizes.
- Build the section header table.
- Assign the sections raw-pointer value for non-virtual sections.

Since the .bss section is virtual, writing the header table is enough to
add support. Writing of a sections raw data, or of any relocations is
not included in this patch.

Testing is done by dumping the section header table, but it needs to be
extended to include dumping the symbol table once readobj support for
dumping auxiallary entries lands.

Differential Revision: https://reviews.llvm.org/D65159

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369454 91177308-0d34-0410-b5e6-96231b3b80d8