Enable SSE 4.2 unconditionally

[alexcrichton]

Fuzzing over the weekend found that i64x2 comparison operators
require pcmpgtq which is an SSE 4.2 instruction. Along the lines of #3816
this commit unconditionally enables and requires SSE 4.2 for compilation
and fuzzing. It will no longer be possible to create a compiler for
x86_64 with simd enabled if SSE 4.2 is disabled.

[cfallin]

Update comment here? s/4.1/4.2/

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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
  it.

  Our documentation should be of the following form:

  Short, simple summary sentence.
  
  More details. These details can be multiple paragraphs. There should be
  information about not just the method, but its parameters and results as
  well.
  
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  Can this method panic? If so, when does it panic?
  
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  Optional example here.
  

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