[MCAssembler] Allow backend to finalize layout post-relaxation.

Differential revision: http://reviews.llvm.org/D19429

llvm-svn: 267796
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index ca8ccb4..4bb0dda 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -914,4 +914,5 @@
   for (unsigned int i = 0, n = Layout.getSectionOrder().size(); i != n; ++i) {
     Layout.getFragmentOffset(&*Layout.getSectionOrder()[i]->rbegin());
   }
+  getBackend().finishLayout(*this, Layout);
 }