Introduce Starboard snapshot writer

Cobalt is a V8 embedder and it has its own platform abstraction. So
to V8 the OS name is "Starboard" instead of Win/Linux/Android even
though the real OS is one of them. To select the right embedded file
writer for Cobalt, we need some customizations in
platform-embedded-file-writer-base.*.

Bug: v8:10927
Change-Id: I6dadb4690ade0b4aebec14bc87fdc6d71c03b3bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2963804
Auto-Submit: John Xu <johnx@google.com>
Commit-Queue: John Xu <johnx@google.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75170}

src/snapshot/embedded/platform-embedded-file-writer-base.cc

@@ -141,6 +141,8 @@ EmbeddedTargetOs ToEmbeddedTargetOs(const char* s) {
     return EmbeddedTargetOs::kMac;
   } else if (string == "win") {
     return EmbeddedTargetOs::kWin;
+  } else if (string == "starboard") {
+    return EmbeddedTargetOs::kStarboard;
   } else {
     return EmbeddedTargetOs::kGeneric;
   }
@@ -153,6 +155,27 @@ std::unique_ptr<PlatformEmbeddedFileWriterBase> NewPlatformEmbeddedFileWriter(
   auto embedded_target_arch = ToEmbeddedTargetArch(target_arch);
   auto embedded_target_os = ToEmbeddedTargetOs(target_os);
 
+  if (embedded_target_os == EmbeddedTargetOs::kStarboard) {
+    // target OS is "Starboard" for all starboard build so we need to
+    // use host OS macros to decide which writer to use.
+    // Cobalt also has Windows-based Posix target platform,
+    // in which case generic writer should be used.
+    switch (DefaultEmbeddedTargetOs()) {
+      case EmbeddedTargetOs::kMac:
+#if defined(V8_TARGET_OS_WIN)
+      case EmbeddedTargetOs::kWin:
+        // V8_TARGET_OS_WIN is used to enable WINDOWS-specific assembly code,
+        // for windows-hosted non-windows targets, we should still fallback to
+        // the generic writer.
+#endif
+        embedded_target_os = DefaultEmbeddedTargetOs();
+        break;
+      default:
+        // In the block below, we will use WriterGeneric for other cases.
+        break;
+    }
+  }
+
   if (embedded_target_os == EmbeddedTargetOs::kAIX) {
     return std::make_unique<PlatformEmbeddedFileWriterAIX>(embedded_target_arch,
                                                            embedded_target_os);

src/snapshot/embedded/platform-embedded-file-writer-base.h

@@ -32,6 +32,7 @@ enum class EmbeddedTargetOs {
   kFuchsia,
   kMac,
   kWin,
+  kStarboard,
   kGeneric,  // Everything not covered above falls in here.
 };