[snapshot] Emit embedded blob code data start as function symbol

The UMA sampling profiler needs a way to detect the beginning of the
embedded builtins code range; a plain symbol is not enough, but a
function symbol should be good.

This changes (x64)

 v8_Default_embedded_blob_code_data_:

to

 v8_Default_embedded_blob_code_data_:
 .type v8_Default_embedded_blob_code_data_, @function
 .size v8_Default_embedded_blob_code_data_, 0

Bug: v8:6666
Change-Id: I5dce3aaca0f36e15ad4339e20fbbdc76282a289e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2450058
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70330}

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

@@ -192,7 +192,14 @@ class EmbeddedFileWriter : public EmbeddedFileWriterInterface {
     w->Comment("instruction streams.");
     w->SectionText();
     w->AlignToCodeAlignment();
-    w->DeclareLabel(EmbeddedBlobCodeDataSymbol().c_str());
+
+    // UMA needs an exposed function-type label at the start of the embedded
+    // code section, thus this label is declared as a function (otherwise we
+    // could use DeclareLabel).
+    static constexpr int kDummyFunctionLength = 0;
+    w->DeclareFunctionBegin(EmbeddedBlobCodeDataSymbol().c_str(),
+                            kDummyFunctionLength);
+    w->DeclareFunctionEnd(EmbeddedBlobCodeDataSymbol().c_str());
 
     for (int i = 0; i < i::Builtins::builtin_count; i++) {
       if (!blob->ContainsBuiltin(i)) continue;