Process two 32-bit digits as one 64-bit digit in all bigint intrinsics on ARM64.

runtime/vm/assembler_arm64.h

Index: runtime/vm/assembler_arm64.h
===================================================================
--- runtime/vm/assembler_arm64.h	(revision 42607)
+++ runtime/vm/assembler_arm64.h	(working copy)
@@ -680,6 +680,9 @@
   void smulh(Register rd, Register rn, Register rm) {
     EmitMiscDP3Source(SMULH, rd, rn, rm, R0, kDoubleWord);
   }
+  void umulh(Register rd, Register rn, Register rm) {
+    EmitMiscDP3Source(UMULH, rd, rn, rm, R0, kDoubleWord);
+  }
   void umaddl(Register rd, Register rn, Register rm, Register ra) {
     EmitMiscDP3Source(UMADDL, rd, rn, rm, ra, kDoubleWord);
   }