There are three ways to parse /proc/self/maps in platform-linux.cc,
remove one to use common code. In the process, add a unit test, and fix
some issues in the latest iteration of /proc/self/maps parsing.

Change-Id: I4701ea49fe8cce53aea0179e194dc48fbebb2ff5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3605226
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80258}