update v8 to 12.4

[garrettgu10]

[fhanau]

Looks like this is failing because the bazel rule for building torque is not working properly – perhaps https://chromium-review.googlesource.com/c/v8/v8/+/5339896 is responsible – not changing v8root based on whether V8 is built from another bazel workspace might be responsible for bazel not finding the torque output files.

[fhanau]

@garrettgu10 for the patch to be applied make sure to add it to the list of v8 patches in WORKSPACE.