Adding dllimport, dllexport and external weak linkage types.

DLL* linkages got full (I hope) codegeneration support in C & both x86
assembler backends.
External weak linkage added for future use, we don't provide any
codegeneration, etc. support for it.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30374 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ExecutionEngine/ExecutionEngine.cpp

@@ -657,7 +657,9 @@ void ExecutionEngine::emitGlobals() {
         }
         
         // If the existing global is strong, never replace it.
-        if (GVEntry->hasExternalLinkage())
+        if (GVEntry->hasExternalLinkage() ||
+            GVEntry->hasDLLImportLinkage() ||
+            GVEntry->hasDLLExportLinkage())
           continue;
         
         // Otherwise, we know it's linkonce/weak, replace it if this is a strong