Update the LTO GoldPlugin documentation

 * Update build instructions to reflect the current source tree layout.
 * Don't inflict CVS on readers; there's a perfectly good git mirror.
 * configure with --disable-werror making it possible to build using clang.
 * ar and nm-new now support the -plugin option.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196069 91177308-0d34-0410-b5e6-96231b3b80d8