Switch iOS gen_snapshot from multi-arch binary to multiple binaries (#37445)

* Switch iOS gen_snapshot from multi-arch binary to multiple binaries

* Make snapshotterPath mutable

packages/flutter_tools/lib/src/base/build.dart

@@ -54,15 +54,12 @@ class GenSnapshot {
       ...additionalArgs,
     ];
 
-    final String snapshotterPath = getSnapshotterPath(snapshotType);
+    String snapshotterPath = getSnapshotterPath(snapshotType);
 
-    // iOS gen_snapshot is a multi-arch binary. Running as an i386 binary will
-    // generate armv7 code. Running as an x86_64 binary will generate arm64
-    // code. /usr/bin/arch can be used to run binaries with the specified
-    // architecture.
+    // iOS has a separate gen_snapshot for armv7 and arm64 in the same,
+    // directory. So we need to select the right one.
     if (snapshotType.platform == TargetPlatform.ios) {
-      final String hostArch = iosArch == IOSArch.armv7 ? '-i386' : '-x86_64';
-      return runCommandAndStreamOutput(<String>['/usr/bin/arch', hostArch, snapshotterPath, ...args]);
+      snapshotterPath += '_' + getNameForIOSArch(iosArch);
     }
 
     StringConverter outputFilter;