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 41816)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -546,6 +546,7 @@
   void XorImmediate(Register dst, const Immediate& imm, Register pp);
 
   void addl(Register dst, Register src);
+  void addl(Register dst, const Immediate& imm);
   void addl(Register dst, const Address& address);
   void addl(const Address& address, Register src);
   void adcl(Register dst, Register src);
@@ -557,6 +558,9 @@
   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();
@@ -576,6 +580,7 @@
   void MulImmediate(Register reg, const Immediate& imm, Register pp);
 
   void subl(Register dst, Register src);
+  void subl(Register dst, const Immediate& imm);
   void subl(Register dst, const Address& address);
   void sbbl(Register dst, Register src);
   void sbbl(Register dst, const Immediate& imm);
@@ -586,6 +591,9 @@
   void subq(Register reg, const Address& address);
   void subq(const Address& address, Register reg);
   void subq(const Address& address, const Immediate& imm);
+  void sbbq(Register dst, Register src);
+  void sbbq(Register dst, const Immediate& imm);
+  void sbbq(Register dst, const Address& address);
 
   void shll(Register reg, const Immediate& imm);
   void shll(Register operand, Register shifter);