Valgrind crashes macOS on arm64

[LouisBrunner]

As discussed #56 (comment), Valgrind suffers from some kind of memory issues on arm64, causing the whole system to slow down and/or crash.

Click here if you want to use Valgrind on arm64 right now, even though it might crash or even damage your computer

Because of the above reason, arm64 support is hidden behind an opt-in environment variable. Make sure you are aware of the risk of using such an experimental version of Valgrind.
As stated in the Valgrind license (which applies to this fork as well), we accept no liability for any damage (be it a loss of data, damage to your computer, loss of warranty or anything else) resulting from your use of Valgrind.

Click here to acknowledge you are aware of the risks and accept that the maintainers of Valgrind and this fork are not liable for any damage to your computer

If you want to use Valgrind on arm64, you will need to set the environment variable `I_ACKNOWLEDGE_THIS_MIGHT_CRASH_OR_DAMAGE_MY_COMPUTER` to a value of `yes`. This needs to be done while building (during `./configure` to be exact) and while running Valgrind itself.

This issue is dedicated to discussion and resolution of this specific memory issue and not to general arm64 support or related issues (e.g. suppressions). If you want to report other issues with arm64, please create a new issue. Any off-topic comments will be mark as such and hidden.

Once this issue is resolved, the opt-in will be removed and arm64 will be available without extra flag.