Implement leaf runtime call stub on ARM and corresponding call redirection

runtime/vm/assembler_arm.cc

Index: runtime/vm/assembler_arm.cc
===================================================================
--- runtime/vm/assembler_arm.cc	(revision 19723)
+++ runtime/vm/assembler_arm.cc	(working copy)
@@ -1398,10 +1398,21 @@
 
 void Assembler::Branch(const ExternalLabel* label, Condition cond) {
   LoadImmediate(IP, label->address(), cond);  // Address is never patched.
-  mov(PC, ShifterOperand(IP), cond);
+  bx(IP, cond);
 }
 
 
+void Assembler::BranchPatchable(const ExternalLabel* label) {
+  // Use a fixed size code sequence, since a function prologue may be patched
+  // with this branch sequence.
+  // Contrarily to BranchLinkPatchable, BranchPatchable requires an instruction
+  // cache flush upon patching.
+  movw(IP, Utils::Low16Bits(label->address()));
+  movt(IP, Utils::High16Bits(label->address()));
+  bx(IP);
+}
+
+
 void Assembler::BranchLink(const ExternalLabel* label) {
   LoadImmediate(IP, label->address());  // Target address is never patched.
   blx(IP);  // Use blx instruction so that the return branch prediction works.