[yaml2obj] - Allow custom fields for the SHT_UNDEF sections.

This is a follow-up refactoring patch for recently
introduced functionality which which reduces the code duplication
and also makes possible to redefine all possible fields of
the first SHT_NULL section (previously it was only possible to set
sh_link and sh_size).

Differential revision: https://reviews.llvm.org/D65140

llvm-svn: 366894