Add getpid in wasi

[youknowone]

Add getpid from wasi-libc. getpid in wasi itself doesn't make sense, but wasi-libc defines an emulated function which returns a constant value.

I found semver/wasi.txt doesn't exist. Do I need to create it?

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[youknowone]

@JohnTitor I am really sorry. This one need to be reverted.

rust-lang/rust#124554 (comment)

The function is not placed in libc.a

[JohnTitor]

Alright 👍, created a revert PR: #3679