[builtins] Declare builtin symbols

This is an attempt to make builtin symbols visible in crash stack
traces. Can be reverted if unsuccessful.

TBR=yangguo@chromium.org

Bug: v8:6666, v8:7722
Change-Id: I74a44b23d1a39d8885992f73d7ed02baf43cfa30
Reviewed-on: https://chromium-review.googlesource.com/1170830Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55054}

src/snapshot/mksnapshot.cc

@@ -190,9 +190,13 @@ class SnapshotWriter {
       // stream.
       if (is_default_variant) {
         // Create nicer symbol names for the default mode.
+        fprintf(fp, "__asm__(\"\" V8_ASM_DECLARE(\"Builtins_%s\"));\n",
+                i::Builtins::name(i));
         fprintf(fp, "__asm__(V8_ASM_LABEL(\"Builtins_%s\"));\n",
                 i::Builtins::name(i));
       } else {
+        fprintf(fp, "__asm__(\"\" V8_ASM_DECLARE(\"%s_Builtins_%s\"));\n",
+                embedded_variant, i::Builtins::name(i));
         fprintf(fp, "__asm__(V8_ASM_LABEL(\"%s_Builtins_%s\"));\n",
                 embedded_variant, i::Builtins::name(i));
       }