llvm-mc: Use Target::createNullStreamer to fix crashes on target-specific asm directives.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229798 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/ARM/null-streamer.ll

@@ -5,3 +5,5 @@ define i32 @main()  {
 entry:
   ret i32 0
 }
+
+module asm ".fnstart"

tools/llvm-mc/llvm-mc.cpp

@@ -465,7 +465,7 @@ int main(int argc, char **argv) {
                                            MAB, ShowInst));
 
   } else if (FileType == OFT_Null) {
-    Str.reset(createNullStreamer(Ctx));
+    Str.reset(TheTarget->createNullStreamer(Ctx));
   } else {
     assert(FileType == OFT_ObjectFile && "Invalid file type!");
     MCCodeEmitter *CE = TheTarget->createMCCodeEmitter(*MCII, *MRI, *STI, Ctx);