Re-enable SIMD when Julia supports ISA-specific code

[jakobnissen]

I've come to the conclusion that Julia does not make it possible to write code that is instruction set architechture (ISA)-specific, i.e. which uses CPU instructions directly.
More specifically, there is no robust way of knowing if Julia is allowed to output any particular instruction.

If Julia ever supports this properly, the SIMD-specific code is still present in v1 onwards, and can be re-enabled. Look for the commit that disabled the SIMD changes ("Disable SIMD capability") and revert it.