Use AVX if available

[eggrobin]

Because of different alignment requirements and VEX encoding/VZEROUPPER woes, and because it would be so low-level and pervasive (we cannot gate every operation on R3Element behind a CPUID test) we need a separate DLL for that, with an intermediate layer that chooses which one to use based on CPUID.

We could generate the intermediate layer from journal.proto, and LoadLibrary/GetProcAddress etc.

This blocks FMA usage on Linux & macOS, see #3010 (comment).

[Growflavor]

just curious: is this relevant only for Linux & macOS? Or is there something here that might also be useful as an alternative to FMA3 on Windows 10 machines with older chips? For users like me who are stuck (for a few more years) with Ivy-Bridge processors that support AVX but not FMA3/AVX2: