Pass CPU string to LTO pipeline.

Previously an empty CPU string was passed to the LTO engine which
resulted in a generic CPU for which certain features like NOPL were
disabled. This fixes that.

Patch by Pratik Bhatu!

llvm-svn: 323801
diff --git a/lld/ELF/LTO.cpp b/lld/ELF/LTO.cpp
index 03628fa..3710bef 100644
--- a/lld/ELF/LTO.cpp
+++ b/lld/ELF/LTO.cpp
@@ -87,6 +87,7 @@
   Conf.DisableVerify = Config->DisableVerify;
   Conf.DiagHandler = diagnosticHandler;
   Conf.OptLevel = Config->LTOO;
+  Conf.CPU = GetCPUStr();
 
   // Set up a custom pipeline if we've been asked to.
   Conf.OptPipeline = Config->LTONewPmPasses;