ART: Move code from RA to T9 in Mips64 longjump

Do not use the return address. It should not be clobbered as we
may need it, e.g., for single-frame deopt.

Change-Id: I401d7b0034bc4e210bda231ec78ee31242053b36
diff --git a/runtime/arch/mips64/context_mips64.cc b/runtime/arch/mips64/context_mips64.cc
index 6637c37..cc6dc7e 100644
--- a/runtime/arch/mips64/context_mips64.cc
+++ b/runtime/arch/mips64/context_mips64.cc
@@ -29,10 +29,10 @@
   std::fill_n(gprs_, arraysize(gprs_), nullptr);
   std::fill_n(fprs_, arraysize(fprs_), nullptr);
   gprs_[SP] = &sp_;
-  gprs_[RA] = &ra_;
+  gprs_[T9] = &t9_;
   // Initialize registers with easy to spot debug values.
   sp_ = Mips64Context::kBadGprBase + SP;
-  ra_ = Mips64Context::kBadGprBase + RA;
+  t9_ = Mips64Context::kBadGprBase + T9;
 }
 
 void Mips64Context::FillCalleeSaves(const StackVisitor& fr) {