Bug 265469

rdar://118890976

It's rare, but possible for two threads to initialize the ARM64 disassembler in JSC at the same time. If this happens, we run into problems in the following code:

    if (!opcodeTable[opcodeGroupNumber])
        opcodeTable[opcodeGroupNumber] = newOpcodeGroup;
    else
        lastGroups[opcodeGroupNumber]->setNext(newOpcodeGroup);
    lastGroups[opcodeGroupNumber] = newOpcodeGroup;

This code builds an array of linked lists of opcode groups. We can get an interleaving where:
  - Thread 1 hits the if statement, and sees the opcode table entry is null.
  - Thread 1 sets the opcode table entry to a new, non-null group.
  - Thread 2 hits the if statement, and sees the opcode table entry is now non-null.
  - Thread 2 continues to setNext on an element of lastGroups, but it's still null at this point! So we crash.

To fix this, we should just protect this initialization function with a lock.