Adds Runtime call stub and Dart entry stub to ARM64.

runtime/vm/constants_arm64.h

Index: runtime/vm/constants_arm64.h
===================================================================
--- runtime/vm/constants_arm64.h	(revision 35156)
+++ runtime/vm/constants_arm64.h	(working copy)
@@ -52,6 +52,13 @@
   SP = 32,
   ZR = 33,
 
+  // SPREG is used from architecture independent code. It is only ever passed to
+  // Simulator::get_register(). Calls to get_register() from architecture
+  // independent code don't include an R31Type, so if we were to define
+  // SPREG to be R31, we wouldn't be able to distinguish from ZR. Therefore we
+  // use another value, and check for it in get_register().
+  SPREG = 34,
+
   // Aliases.
   IP0 = R16,
   IP1 = R17,
@@ -111,7 +118,6 @@
 const Register TMP2 = R17;
 const Register CTX = R28;  // Caches current context in generated code.
 const Register PP = R27;  // Caches object pool pointer in generated code.
-const Register SPREG = R31;  // Stack pointer register.
 const Register FPREG = FP;  // Frame pointer register.
 const Register ICREG = R5;  // IC data register.
 
@@ -141,8 +147,8 @@
 const RegList kAbiPreservedCpuRegs =
     (1 << R19) | (1 << R20) | (1 << R21) | (1 << R22) |
     (1 << R23) | (1 << R24) | (1 << R25) | (1 << R26) |
-    (1 << R27) | (1 << R28) | (1 << R29);
-const int kAbiPreservedCpuRegCount = 11;
+    (1 << R27) | (1 << R28);
+const int kAbiPreservedCpuRegCount = 10;
 const VRegister kAbiFirstPreservedFpuReg = V8;
 const VRegister kAbiLastPreservedFpuReg = V15;
 const int kAbiPreservedFpuRegCount = 8;
@@ -585,6 +591,8 @@
   static const int32_t kBreakPointInstruction =  // hlt #kImmExceptionIsDebug.
       HLT | (kImmExceptionIsDebug << kImm16Shift);
   static const int kBreakPointInstructionSize = kInstrSize;
+  static const int32_t kRedirectInstruction =
+      HLT | (kImmExceptionIsRedirectedCall << kImm16Shift);
 
   // Read one particular bit out of the instruction bits.
   inline int Bit(int nr) const {