Quick notes on `exec` discussion

[Yaxuan-w]

Quick notes on exec discussion with @qianxichen233 @rennergade @yzhang71. We're going to fulfill with details in the future.

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cancelstatus -- need to dig into when fixing the exit issue

pthread id is stored here
-- main thread id is stored (like when we create different threads, we store the main thread id to handle signal etc.)

1. everything exit correct. Small interface(..?) and return to wasmtime
2. call kernel pthread_cancel and then handle situations in wasmtime
   Those problems should related to thread (exit) issue we've talked before.

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rev_shm lookup table for shm
// fork(): preserved
// exec(): clear

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futex -- create futex table save address of futexes
handle different in fork and exec

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remove cv/mutex/semaphore tables
one mutex table to ensure behaviors in fork and exec

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maybe thread scheduling in

signal staffs -- portable?