Add a function to de-initialize the environment-variable state.

Add a function to de-initialize the environment-variable state.

Add a `__wasilibc_deinit_environ` function which clears the current
environment variable state to the state where next time the environment
variable functions are called, they'll reinitialize the environment.

And add a `__wasilibc_maybe_reinitialize_environ_eagerly` function to
reinitialize the environment variable state if `environ` or `_environ`
are needed.

These functions are needed by wizer to be able to suspend and resume
a program and have it read new environment variables from the host
environment; see bytecodealliance/wizer#8 for background.