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

runtime/vm/assembler_mips.h

Index: runtime/vm/assembler_mips.h
===================================================================
--- runtime/vm/assembler_mips.h	(revision 25620)
+++ runtime/vm/assembler_mips.h	(working copy)
@@ -145,10 +145,11 @@
 
 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),
         delay_slot_available_(false),
         in_delay_slot_(false),
         comments_() { }
@@ -169,6 +170,8 @@
     buffer_.FinalizeInstructions(region);
   }
 
+  bool use_far_branches() const { return use_far_branches_; }
+
   // Set up a stub frame so that the stack traversal code can easily identify
   // a stub frame.
   void EnterStubFrame(bool uses_pp = false);
@@ -1094,6 +1097,7 @@
   GrowableObjectArray& object_pool_;  // Objects and patchable jump targets.
   int prologue_offset_;
 
+  bool use_far_branches_;
   bool delay_slot_available_;
   bool in_delay_slot_;