The infrastructure runs everything already in Python3, so this is
mostly a clean-up.

For MB, a python2 holdover was removed and new lint errors were
fixed.

The renames were automated with:
git grep -e "/usr/bin/python$" |
  cut -d':' -f1 |
  xargs
  sed -i 's/#!\/usr\/bin\/python$/#!\/usr\/bin\/python3/1'

and
git grep -e "/usr/bin/env python$" |
  cut -d':' -f1 |
  xargs
  sed -i 's/#!\/usr\/bin\/env python$/#!\/usr\/bin\/env python3/1'

Bug: v8:13148
Change-Id: If4f3c7635e72fa134798d55314ac1aa92ddd01bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3811499Reviewed-by: Liviu Rau <liviurau@google.com>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82231}

Bug: v8:9871
Change-Id: I4084771cfc4d34f8e1b9d5265e115f9eac1098d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2646165Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Gus Caplan <snek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72293}

While clang++ doesn't mind, g++ complains about apostrophes in #error
messages as they appear to be unterminated single-quoted literals. So
add surrounding double-quotes.

Bug: v8:11292
Change-Id: Ib263cb20c73650348599992186a0e7de11b1b306
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2639763
Auto-Submit: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72192}

Adds a v8-gn.h file containing defines that are used in the
externally-visible headers files like v8.h. This must be included by
include/v8config.h which includes it if the GN flag
v8_generate_external_defines_header is on. (Currently off by default).

To enable the v8config.h file to be included without the other v8
headers (as required by cppgc), this moves it into its own header set
which sets up the include path correctly.

Also updates some headers to ensure v8config.h is included before using
externally-visible defines.

Bug: v8:11292
Change-Id: I5be634f4adfbef144bf684071461d64f1cb30899
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2608212
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72140}