A third attempt to provide correct conversion specifiers on MacOS.

runtime/vm/disassembler_x64.cc

Index: runtime/vm/disassembler_x64.cc
diff --git a/runtime/vm/disassembler_x64.cc b/runtime/vm/disassembler_x64.cc
index 9d38571ba710b2a0c4e8af5ea5facfe9e5d43828..3460347b6d73bf310feba1ba74347ca3c78c6238 100644
--- a/runtime/vm/disassembler_x64.cc
+++ b/runtime/vm/disassembler_x64.cc
@@ -559,7 +559,7 @@ int DisassemblerX64::PrintImmediate(uint8_t* data, OperandSize size) {
       value = 0;  // Initialize variables on all paths to satisfy the compiler.
       count = 0;
   }
-  AppendToBuffer("%#"Px"", value);
+  AppendToBuffer("%#"Px64"", value);
   return count;
 }
 
@@ -1755,7 +1755,7 @@ int DisassemblerX64::InstructionDecode(uword pc) {
           default:
             UNREACHABLE();
         }
-        AppendToBuffer("test%c rax,%#"Px"",
+        AppendToBuffer("test%c rax,%#"Px64"",
                        operand_size_code(),
                        value);
         break;