Adds native/leaf runtime call stub and redirection on MIPS.

runtime/vm/assembler_mips.h

Index: runtime/vm/assembler_mips.h
===================================================================
--- runtime/vm/assembler_mips.h	(revision 20794)
+++ runtime/vm/assembler_mips.h	(working copy)
@@ -515,6 +515,15 @@
     jr(TMP);
   }
 
+  void BranchPatchable(const ExternalLabel* label) {
+    const uint16_t low = Utils::Low16Bits(label->address());
+    const uint16_t high = Utils::High16Bits(label->address());
+    lui(TMP, Immediate(high));
+    ori(TMP, TMP, Immediate(low));
+    jr(TMP);
+    delay_slot()->nop();

[regis, 2013/04/02 20:40:51]

This works, but there is already a nop in the delay slot. This would be
clearer/simpler:
delay_slot_available_ = false;  // CodePatcher expects a nop.

+  }
+
   void BranchLink(const ExternalLabel* label) {
     LoadImmediate(TMP, label->address());
     jalr(TMP);
@@ -525,6 +534,7 @@
         Array::data_offset() + 4*AddExternalLabel(label) - kHeapObjectTag;
     LoadWordFromPoolOffset(TMP, offset);
     jalr(TMP);
+    delay_slot()->nop();

[regis, 2013/04/02 20:40:51]

ditto

   }
 
   // If the signed value in rs is less than value, rd is 1, and 0 otherwise.