Set ABI if it hasn't been set on the command line.
Check if architecture & ABI combination is valid.

llvm-svn: 140230
diff --git a/llvm/lib/Target/Mips/MipsSubtarget.h b/llvm/lib/Target/Mips/MipsSubtarget.h
index c6b43a7..d9dddad 100644
--- a/llvm/lib/Target/Mips/MipsSubtarget.h
+++ b/llvm/lib/Target/Mips/MipsSubtarget.h
@@ -29,7 +29,7 @@
 public:
   // NOTE: O64 will not be supported.
   enum MipsABIEnum {
-    O32, N32, N64, EABI
+    UnknownABI, O32, N32, N64, EABI
   };
 
 protected: