[z/OS] Implement shared memory handling for JIT (#89933)

Fix 'use of undeclared identifier' build errors for shm_ functions on
z/OS by implementing the functionality using shmget(), shmat(), and
shmdt(). Use the BLAKE3 hash to map the name of the shared memory to a
key.

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Co-authored-by: Kai Nacke <kai.peter.nacke@ibm.com>