Preliminary changes to allow mips target to build.

It compiles, but it doesn't work yet.

Change-Id: I2973a03bd956d8d398b9cfd1047e66fbf3ff439c
diff --git a/src/compiler/codegen/mips/MipsRallocUtil.cc b/src/compiler/codegen/mips/MipsRallocUtil.cc
index ded59f0..43fcc07 100644
--- a/src/compiler/codegen/mips/MipsRallocUtil.cc
+++ b/src/compiler/codegen/mips/MipsRallocUtil.cc
@@ -149,8 +149,9 @@
 
 extern RegLocation oatGetReturnAlt(CompilationUnit* cUnit)
 {
-  UNIMPLEMENTED(FATAL);
-  RegLocation res = LOC_C_RETURN;
+  RegLocation res = LOC_C_RETURN_ALT;
+  oatClobber(cUnit, res.lowReg);
+  oatMarkInUse(cUnit, res.lowReg);
   return res;
 }