Reland "[build] Enable external flag header by default"

This is a reland of d1da9694

Relanding now that fixes for perfetto and cppgc are in:
https://chromium-review.googlesource.com/c/v8/v8/+/2640458
https://chromium-review.googlesource.com/c/v8/v8/+/2640480

Original change's description:
> [build] Enable external flag header by default
>
> Turns on v8_generate_external_defines_header.
>
> Bug: v8:11292
> Change-Id: I4b1d9b47390b560b7cbf677948310694d8b03367
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2610966
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Commit-Queue: Dan Elphick <delphick@chromium.org>
> Auto-Submit: Dan Elphick <delphick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#72201}

Bug: v8:11292
Change-Id: Ia47eeb6c45f4cc3db72c10782d677b69506fa3d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2642249Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72226}

BUILD.gn

@@ -288,7 +288,7 @@ declare_args() {
 
   # If enabled then macro definitions that are used in externally visible
   # header files are placed in a separate header file v8-gn.h.
-  v8_generate_external_defines_header = false
+  v8_generate_external_defines_header = true
 }
 
 # Derived defaults.