Define __ELF__ for linux systems.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64553 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 9e81ed7..b37939f 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -85,6 +85,7 @@
   Define(Defs, "__linux");
   Define(Defs, "linux");
   Define(Defs, "__gnu_linux__");
+  Define(Defs, "__ELF__", "1");
 }
 
 /// getDarwinNumber - Parse the 'darwin number' out of the specific targe