[gn build] Add build files for obj2yaml, yaml2obj, and lib/ObjectYAML

The two executables are needed by check-lld.

Differential Revision: https://reviews.llvm.org/D55687


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349290 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed