Process two 32-bit digits as one 64-bit digit in bigint absAdd intrinsic on x64.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 41744)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -557,6 +557,9 @@
   void addq(Register dst, const Address& address);
   void addq(const Address& address, const Immediate& imm);
   void addq(const Address& address, Register src);
+  void adcq(Register dst, Register src);
+  void adcq(Register dst, const Immediate& imm);
+  void adcq(Register dst, const Address& address);
 
   void cdq();
   void cqo();