dmd Arm64 - first function compile!

[solidstate1991]

Once I was looking into getting it done, but every time I had to 
realize it was way out of scope of my capabilities.

Once it's feasible enough, I'll be testing my libraries with DMD 
too on my Raspberry Pi, even though there's only a minimal 
difference between ARM and x86-64 once properly compiled. Biggest 
one boils down to vector optimizations, which is likely a quirk 
of LDC2.