MIPS64: Add big-endian support for mips64.

src/mips64/constants-mips64.h

Index: src/mips64/constants-mips64.h
diff --git a/src/mips64/constants-mips64.h b/src/mips64/constants-mips64.h
index 898a4dbb1d19418e2a767646cc03b4c3a1c13395..3e427ecf359c892fea1b49c5e6a9dd4a7db70fe7 100644
--- a/src/mips64/constants-mips64.h
+++ b/src/mips64/constants-mips64.h
@@ -35,6 +35,16 @@ enum ArchVariants {
 #endif
 
 
+  enum Endianness { kLittle, kBig };
+
+#if defined(V8_TARGET_LITTLE_ENDIAN)
+  static const Endianness kArchEndian = kLittle;
+#elif defined(V8_TARGET_BIG_ENDIAN)
+  static const Endianness kArchEndian = kBig;
+#else
+#error Unknown endianness
+#endif
+
 // TODO(plind): consider deriving ABI from compiler flags or build system.
 
 // ABI-dependent definitions are made with #define in simulator-mips64.h,