fix(caching_2to3): multiprocessing needs read/write perms on /dev/shm

Specifically semaphores used in multiprocessing use POSIX semaphores on
Linux that require read/write/execute access to shared memory
(/dev/shm).

This fixes the case where the multiprocessing implementation for 2to3
caching was selected but /dev/shm didn't have correct permissions to
work correctly (see Gentoo bugs 514948 and 480578).