Add fallback SIMD implementation

[taylordotfish]

Currently, on platforms that don't have an implementation of SIMDNativeOps, SIMDRegister is not available.

This PR makes functionality gracefully degrade by adding a fallback implementation of SIMDNativeOps that simply calls the fallback functions that already exist in SIMDFallbackOps.

On platforms that already support SIMD, nothing changes, but other platforms will now have implementations of SIMDRegister that use the fallback functions, so software that uses SIMDRegister will automatically support more platforms without any additional effort.

All unit tests pass on x86_64, aarch64, and ppc64le.