MIPS64: Fix stub_test implementation

Clang doesn't recognize f0-f31 registers without '$' symbol in
inline assembly. So now they are $f0-$f31.

This change allows building ART tests.

Change-Id: Ie08e1a5aceee266f7b674c9883bed53473696178
diff --git a/runtime/arch/stub_test.cc b/runtime/arch/stub_test.cc
index afa48cd..8746bad 100644
--- a/runtime/arch/stub_test.cc
+++ b/runtime/arch/stub_test.cc
@@ -431,9 +431,9 @@
           [referrer] "r"(referrer), [hidden] "r"(hidden)
         : "at", "v0", "v1", "t0", "t1", "t2", "t3", "s0", "s1", "s2", "s3", "s4", "s5", "s6", "s7",
           "t8", "t9", "k0", "k1", "fp", "ra",
-          "f0", "f1", "f2", "f3", "f4", "f5", "f6", "f7", "f8", "f9", "f10", "f11", "f12", "f13",
-          "f14", "f15", "f16", "f17", "f18", "f19", "f20", "f21", "f22", "f23", "f24", "f25", "f26",
-          "f27", "f28", "f29", "f30", "f31",
+          "$f0", "$f1", "$f2", "$f3", "$f4", "$f5", "$f6", "$f7", "$f8", "$f9", "$f10", "$f11",
+          "$f12", "$f13", "$f14", "$f15", "$f16", "$f17", "$f18", "$f19", "$f20", "$f21", "$f22",
+          "$f23", "$f24", "$f25", "$f26", "$f27", "$f28", "$f29", "$f30", "$f31",
           "memory");  // clobber.
 #elif defined(__x86_64__) && !defined(__APPLE__)
 #define PUSH(reg) "pushq " # reg "\n\t .cfi_adjust_cfa_offset 8\n\t"