[SDK] Adds dart2native arguments for gen_snapshot & AOT runtime

Example:
  # sdk (x64)
  $ ./tools/build.py -a x64 -m product create_sdk
  $ DART2NATIVE_X64=out/ProductX64/dart-sdk/bin/dart2native
  ninja -C out/ProductX64 create_sdk
  [...]
  The build took XXX seconds

  # gen_snapshot (simarm)
  $ ./tools/build.py -a simarm -m product copy_gen_snapshot
  ninja -C out/ProductSIMARM copy_gen_snapshot
  [...]
  The build took XXX seconds
  $ GEN_SNAPSHOT_SIMARM=out/ProductSIMARM/dart-sdk/bin/utils/gen_snapshot
  $ file $GEN_SNAPSHOT_SIMARM
  [...]/gen_snapshot: ELF 32-bit LSB executable, Intel 80386, [...]

  # dartaotruntime (arm)
  $ ./tools/build.py -a arm -m product copy_dartaotruntime
  ninja -C out/ProductXARM copy_dartaotruntime
  [...]
  The build took XXX seconds
  $ DARTAOTRUNTIME_XARM=out/ProductXARM/dart-sdk/bin/dartaotruntime
  $ file $DARTAOTRUNTIME_XARM
  [...]/dartaotruntime: ELF 32-bit LSB shared object, ARM, [...]

  # test
  $ cat cpu.dart
  import 'dart:io';

  void main() {
    final lines = File('/proc/cpuinfo').readAsLinesSync();
    final model = lines.firstWhere((line) => line.startsWith('model name'));
    print(model.split(':').last.trim());
  }

  # x64
  $ $DART2NATIVE_X64 cpu.dart -o cpu_x64
  Generated: [...]/cpu_x64
  $ file cpu_x64
  cpu_x64: ELF 64-bit LSB shared object, x86-64, [...]
  $ ./cpu_x64
  Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz

  # arm
  $ $DART2NATIVE_X64 cpu.dart --gen-snapshot $GEN_SNAPSHOT_SIMARM --aot-runtime $DARTAOTRUNTIME_XARM -o cpu_arm
  Generated: [...]/cpu_arm
  $ file cpu_arm
  cpu_arm: ELF 32-bit LSB shared object, ARM, [...]
  # -> rpi
  pi@raspberrypi:~ $ ./cpu_arm
  ARMv7 Processor rev 3 (v7l)

pkg/dart2native/bin/dart2native.dart

@@ -59,6 +59,14 @@ E.g.: dart2native --packages=/tmp/pkgs main.dart
 ''')
     ..addOption('save-debugging-info', abbr: 'S', valueHelp: 'path', help: '''
 Remove debugging information from the output and save it separately to the specified file. <path> can be relative or absolute.
+''')
+    ..addOption('gen-snapshot', valueHelp: 'path', hide: true, help: '''
+Override the gen_snapshot tool. <path> can be relative or absolute.
+E.g.: dart2native main.dart -g /tmp/dart-sdk-simarm/bin/utils/gen_snapshot
+''')
+    ..addOption('aot-runtime', valueHelp: 'path', hide: true, help: '''
+Override the AOT runtime. <path> can be relative or absolute.
+E.g.: dart2native main.dart -r /tmp/dart-sdk-arm/bin/dartaotruntime
 ''')
     ..addOption('enable-experiment',
         defaultsTo: '', valueHelp: 'feature', hide: true, help: '''
@@ -106,6 +114,8 @@ Comma separated list of experimental features.
         enableExperiment: parsedArgs['enable-experiment'],
         enableAsserts: parsedArgs['enable-asserts'],
         verbose: parsedArgs['verbose'],
+        overrideGenSnapshot: parsedArgs['gen-snapshot'],
+        overrideDartaotruntime: parsedArgs['aot-runtime'],
         extraOptions: parsedArgs['extra-gen-snapshot-options']);
   } catch (e) {
     stderr.writeln('Failed to generate native files:');

pkg/dart2native/lib/generate.dart

@@ -27,6 +27,8 @@ Future<void> generateNative({
   String enableExperiment = '',
   bool enableAsserts = false,
   bool verbose = false,
+  String overrideGenSnapshot,
+  String overrideDartaotruntime,
   List<String> extraOptions = const [],
 }) async {
   final Directory tempDir = Directory.systemTemp.createTempSync();
@@ -69,8 +71,13 @@ Future<void> generateNative({
     final String snapshotFile = (outputKind == Kind.aot
         ? outputPath
         : path.join(tempDir.path, 'snapshot.aot'));
-    final snapshotResult = await generateAotSnapshot(genSnapshot, kernelFile,
-        snapshotFile, debugPath, enableAsserts, extraOptions);
+    final snapshotResult = await generateAotSnapshot(
+        overrideGenSnapshot ?? genSnapshot,
+        kernelFile,
+        snapshotFile,
+        debugPath,
+        enableAsserts,
+        extraOptions);
     if (snapshotResult.exitCode != 0) {
       stderr.writeln(snapshotResult.stdout);
       stderr.writeln(snapshotResult.stderr);
@@ -82,7 +89,8 @@ Future<void> generateNative({
       if (verbose) {
         print('Generating executable.');
       }
-      await writeAppendedExecutable(dartaotruntime, snapshotFile, outputPath);
+      await writeAppendedExecutable(
+          overrideDartaotruntime ?? dartaotruntime, snapshotFile, outputPath);
 
       if (Platform.isLinux || Platform.isMacOS) {
         if (verbose) {