Add a checksum to jmp_buf on mips and mips64.

Make it easier to diagnose applications mucking with the contents of
jmp_buf by checksumming its contents.

Bug: http://b/27417786
Change-Id: I473bc2871dece23a9b9d02481945246160d671c6
diff --git a/tests/setjmp_test.cpp b/tests/setjmp_test.cpp
index b7e856f..bb01601 100644
--- a/tests/setjmp_test.cpp
+++ b/tests/setjmp_test.cpp
@@ -254,7 +254,7 @@
 
   if (value == 0) {
     // Flip a bit.
-    reinterpret_cast<long*>(jb)[0] ^= 1;
+    reinterpret_cast<long*>(jb)[1] ^= 1;
 
     EXPECT_DEATH(longjmp(jb, 1), "checksum mismatch");
   } else {