deps: update V8 to 6.4

Floating two patches that have not yet landed upstream:

• https://chromium-review.googlesource.com/c/v8/v8/+/804554
  • Merge request: https://bugs.chromium.org/p/v8/issues/detail?id=7191
• https://chromium-review.googlesource.com/c/v8/v8/+/806774
  • Merge request: https://bugs.chromium.org/p/v8/issues/detail?id=7198

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

V8, build

CI: https://ci.nodejs.org/job/node-test-pull-request/11908/ V8: https://ci.nodejs.org/job/node-test-commit-v8-linux/1101/