Fixes #3648: Move Gentoo mem hooks override to v1.6

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r28702 [[BR]]
Add memory hooks override for Gentoo sandbox v2.5, too.  Thanks to
Justin Bronder for the patch.

r28708 [[BR]]
Followup from Justin Bronder: Looks like I spoke too soon.  The
sandbox team has informed me that they are getting rid of SANDBOX_PID
in the future and that using SANDBOX_ON would be preferred.