Configure Mach ports vs signals via Config

[alexcrichton]

This commit adds a Config::macos_use_mach_ports configuration option to replace the old posix-signals-on-macos compile-time Cargo feature. This'll make Wasmtime a tad larger on macOS but likely negligibly so. Otherwise this is intended to provide a resolution to #6785 where embedders will be able to use any build of Wasmtime and configure at runtime how trap handling should happen.

Functionally this commit additionally registers a pthread_atfork handler to cause any usage of Wasmtime in the child to panic. This should help head off a known-invalid state in case anyone runs into it in the future.

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Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• If you added a new Config method, you wrote extensive documentation for
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[elliottt]

This looks great to me! Thanks for the detailed comment on the ports config!