Enables per-function far-branches for ARM and MIPS.

runtime/vm/assembler_arm.h

Index: runtime/vm/assembler_arm.h
===================================================================
--- runtime/vm/assembler_arm.h	(revision 25638)
+++ runtime/vm/assembler_arm.h	(working copy)
@@ -297,10 +297,11 @@
 
 class Assembler : public ValueObject {
  public:
-  Assembler()
+  explicit Assembler(bool use_far_branches = false)
       : buffer_(),
         object_pool_(GrowableObjectArray::Handle()),
         prologue_offset_(-1),
+        use_far_branches_(use_far_branches),
         comments_() { }
   ~Assembler() { }
 
@@ -317,6 +318,8 @@
   }
   const GrowableObjectArray& object_pool() const { return object_pool_; }
 
+  bool use_far_branches() const { return use_far_branches_; }
+
   void FinalizeInstructions(const MemoryRegion& region) {
     buffer_.FinalizeInstructions(region);
   }
@@ -733,6 +736,8 @@
   GrowableObjectArray& object_pool_;  // Objects and patchable jump targets.
   int32_t prologue_offset_;
 
+  bool use_far_branches_;
+
   int32_t AddObject(const Object& obj);
   int32_t AddExternalLabel(const ExternalLabel* label);