[PECOFF] Add a note about underscore symbol prefix.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@188090 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] Update to use lit_config and lit package, as appropriate.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@188093 91177308-0d34-0410-b5e6-96231b3b80d8