Emit assembly instead of a blob for the instructions snapshot, and provide labels for the entry poi…

tools/precompilation/create_instructions_snapshot_assembly.dart

-import 'dart:io';
-
-void main(List<String> args) {
-  print(args[0]);
-  print(args[1]);
-
-  var bytes = new File(args[0]).readAsBytesSync();
-  print(bytes.length);
-
-  var out = new StringBuffer();
-  out.writeln(".text");
-  out.writeln("  .globl _kInstructionsSnapshot");
-  out.writeln("_kInstructionsSnapshot:");
-  out.writeln("  .balign 32, 0");
-  for (var i = 0; i < bytes.length; i++) {
-    var byte = bytes[i];
-    out.writeln("  .byte $byte");
-  }
-
-  new File(args[1]).writeAsString(out.toString());
-}