Update exec to align with POSIX standard

[Yaxuan-w]

Description

Fixes #33

To fully implement the exec functionality while aligning with POSIX standards.

In our implementation, WASM is responsible for handling functionalities such as loading and executing the new program, preserving process attributes, and resetting memory and the stack. In RawPOSIX, the focus is on memory management inheritance and resource cleanup and release. Specifically, RawPOSIX handles tasks such as clearing memory mappings, resetting shared memory, managing file descriptors (closing or inheriting them based on the should_cloexec flag in fdtable), resetting semaphores, and managing process attributes and threads (terminating unnecessary threads).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

NEED TEST

Checklist:

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been added to a pull request and/or merged in other modules (native-client, lind-glibc, lind-project)

[qianxichen233]

Could you comment out this back for lind-wasm use?