[yaml2elf] - Treat the SHN_UNDEF section as kind of regular section.

We have a logic that adds a few sections implicitly.
Though the SHT_NULL section with section number 0
is an exception.

In D64913 I want to teach yaml2obj to redefine the null section.
And in this patch I add it to the sections list,
to make it kind of a regular section.

Differential revision: https://reviews.llvm.org/D65087

llvm-svn: 366785