MC: Emit COFF section flags in the "proper" order

COFF section flags are not idempotent:
  'rd' will make a read-write section because 'd' implies write
  'dr' will make a read-only section because 'r' disables write

llvm-svn: 228490
diff --git a/llvm/test/CodeGen/ARM/Windows/read-only-data.ll b/llvm/test/CodeGen/ARM/Windows/read-only-data.ll
index 0ccb5ed..0438d68 100644
--- a/llvm/test/CodeGen/ARM/Windows/read-only-data.ll
+++ b/llvm/test/CodeGen/ARM/Windows/read-only-data.ll
@@ -10,6 +10,6 @@
   ret void
 }
 
-; CHECK: .section .rdata,"rd"
+; CHECK: .section .rdata,"dr"
 ; CHECK-NOT: .section ".rodata.str1.1"