Add a method to share Config across machines

[alexcrichton]

With Module::{serialize,deserialize} it should be possible to share
wasmtime modules across machines or CPUs. Serialization, however, embeds
a hash of all configuration values, including cranelift compilation
settings. By default wasmtime's selection of the native ISA would enable
ISA flags according to CPU features available on the host, but the same
CPU features may not be available across two machines.

This commit adds a Config::cranelift_clear_cpu_flags method which
allows clearing the target-specific ISA flags that are automatically
inferred by default for the native CPU. Options can then be
incrementally built back up as-desired with teh cranelift_other_flag
method.

[alexcrichton]

Ah it's also worth mentioning that this commit fixes a bug where differences in native CPU flags weren't taken into account when deserializing blobs in Module::deserialize, just ISA-independent settings. That was fixed with the addition of a new trait method on TargetIsa

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