MC: permit emitting a symbol value as section relative

This adds an optional parameter to the EmitSymbolValue method in MCStreamer to
permit emitting a symbol value as a section relative value.  This is to cover
the use in MCDwarf which should not really know about how to emit a section
relative value for a given target.

This addresses post-review comments from Eric Christopher in SVN r213275.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213463 91177308-0d34-0410-b5e6-96231b3b80d8