[mksnapshot] Add runtime flag to specify the target architecture

The mksnapshot-specific runtime flag --target-arch, together with
--target-os, specifies the target platform for the generated embedded.S
file.

Bug: v8:9103
Change-Id: Icb03a381101e7ab0db4a5fbbf3be8e23ed0b1a1c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1624165
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61739}

BUILD.gn

@@ -1208,6 +1208,7 @@ template("run_mksnapshot") {
       # mksnapshot needs to know which target OS to use at runtime.  It's weird,
       # but the target OS is really |current_os|.
       "--target_os=$current_os",
+      "--target_arch=$current_cpu",
     ]
 
     args += invoker.args

src/flag-definitions.h

@@ -1209,6 +1209,8 @@ DEFINE_STRING(startup_src, nullptr,
               "Write V8 startup as C++ src. (mksnapshot only)")
 DEFINE_STRING(startup_blob, nullptr,
               "Write V8 startup blob file. (mksnapshot only)")
+DEFINE_STRING(target_arch, nullptr,
+              "The mksnapshot target arch. (mksnapshot only)")
 DEFINE_STRING(target_os, nullptr, "The mksnapshot target os. (mksnapshot only)")
 
 //

src/snapshot/embedded-file-writer.h

@@ -155,9 +155,14 @@ class EmbeddedFileWriter : public EmbeddedFileWriterInterface {
   }
 
   void SetEmbeddedVariant(const char* embedded_variant) {
+    if (embedded_variant == nullptr) return;
     embedded_variant_ = embedded_variant;
   }
 
+  void SetTargetArch(const char* target_arch) { target_arch_ = target_arch; }
+
+  void SetTargetOs(const char* target_os) { target_os_ = target_os; }
+
   void WriteEmbedded(const i::EmbeddedData* blob) const {
     MaybeWriteEmbeddedFile(blob);
   }
@@ -473,8 +478,18 @@ class EmbeddedFileWriter : public EmbeddedFileWriterInterface {
   std::map<const char*, int> external_filenames_;
   std::vector<const char*> external_filenames_by_index_;
 
+  // The file to generate or nullptr.
   const char* embedded_src_path_ = nullptr;
+
+  // The variant is only used in multi-snapshot builds and otherwise set to
+  // "Default".
   const char* embedded_variant_ = kDefaultEmbeddedVariant;
+
+  // {target_arch} and {target_os} control the generated assembly format. Note
+  // these may differ from both host- and target-platforms specified through
+  // e.g. V8_OS_* and V8_TARGET_ARCH_* defines.
+  const char* target_arch_ = nullptr;
+  const char* target_os_ = nullptr;
 };
 
 }  // namespace internal

src/snapshot/mksnapshot.cc

@@ -239,9 +239,9 @@ int main(int argc, char** argv) {
 
     i::EmbeddedFileWriter embedded_writer;
     embedded_writer.SetEmbeddedFile(i::FLAG_embedded_src);
-    if (i::FLAG_embedded_variant != nullptr) {
     embedded_writer.SetEmbeddedVariant(i::FLAG_embedded_variant);
-    }
+    embedded_writer.SetTargetArch(i::FLAG_target_arch);
+    embedded_writer.SetTargetOs(i::FLAG_target_os);
 
     std::unique_ptr<char> embed_script(
         GetExtraCode(argc >= 2 ? argv[1] : nullptr, "embedding"));