mmap behaves differently on Windows. This change adjust how grokdump.py
uses it so that the script will run on Windows.

The disassembly doesn't work due to lack of /usr/bin/objdump - fixing
that is out of scope for this change. The output is still useful even
without the disassembly.

Change-Id: I0db2d09e3ed10f0ca666cbbd438fbd82565906d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1866958
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64376}