[yaml2obj] - Lookup relocation symbols in dynamic symbol when .dynsym referenced.

[yaml2obj] - Lookup relocation symbols in dynamic symbol when .dynsym referenced.

This fixes https://bugs.llvm.org/show_bug.cgi?id=40337.

Previously, it was always assumed that relocations referenced symbols in the static symbol table.
Now, if the Link field references a section called ".dynsym" it will look up these symbols
in the dynamic symbol table.

This patch is heavily based on D59097 by James Henderson

Differential revision: https://reviews.llvm.org/D66532

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369645 91177308-0d34-0410-b5e6-96231b3b80d8