Default to Pulley on cranelift/winch-unsupported platforms

[alexcrichton]

This commit updates the selection of the default target used by a Config to take into account the host architecture and possibly use Pulley instead of the host target itself. The goal here is that when a target isn't explicitly configured then Wasmtime is tasked with picking a reasonable default to execute code with. If neither Winch nor Cranelift has any support then the only way to possibly execute code would be with Pulley, so in these situations pulley becomes the default target.

The goal of this change is to make testing easier on 32-bit platforms where there is no compiler support. This means we won't have to update all tests to explicitly configure pulley on unsupported platforms. Additionally this means that an eventual wasmtime executable for 32-bit platforms will use Pulley by default and won't need any extra configuration.

[github-actions]

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