Now the following command builds mkgrokdump for x64.release and runs it
to update v8heapconst.py:

  gm.py mkgrokdump

Building the binary for other architectures still works as before.

No-Try: true
Change-Id: Iacfa1a50702b0452d00ba18e1306423b161ffe65
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2317352
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69054}