* Enable SSE 4.2 unconditionally
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.
* Update comment