Fix simarm build for newer gcc's

runtime/vm/constants_arm.h

Index: runtime/vm/constants_arm.h
diff --git a/runtime/vm/constants_arm.h b/runtime/vm/constants_arm.h
index 89ac511f97cc7afe11ec925bce25cffaa9d3fcde..d1e9bc67fc0bf12b0b1e93462ddb99c0a6952a9c 100644
--- a/runtime/vm/constants_arm.h
+++ b/runtime/vm/constants_arm.h
@@ -260,7 +260,7 @@ static inline SRegister OddSRegisterOf(DRegister d) {
 // Register aliases for floating point scratch registers.
 const QRegister QTMP = Q7;                     // Overlaps with DTMP, STMP.
 const DRegister DTMP = EvenDRegisterOf(QTMP);  // Overlaps with STMP.
-const SRegister STMP = EvenSRegisterOf(DTMP);
+const SRegister STMP DART_USED = EvenSRegisterOf(DTMP);
 
 // Architecture independent aliases.
 typedef QRegister FpuRegister;